I think that this is a rather tough question to answer. One could argue that they went wrong with Rattle & Hum, though they quickly rebounded with Achtung Baby. The next logical dot on their timeline to consider as their going wrong would be Pop: the tour was booked before the album was finished, resulting, in the band's estimation, in the album being undercooked (I agree to a point, in particular regards to bad production on songs like If God Will Send His Angels and If You Wear That Velvet Dress), which resulted in their not having ample time to rehearse for the tour, which we all know resulted in some rather bumpy early shows. On that same tour, though, they booked some rather odd locations, and then wondered why the stadiums were half empty. Take Clemson, South Carolina as an example. Clemson is emphatically a quintessential college town. Booking a stadium show during non-school season, and then wondering why a 50,000+ stadium barely sells half of the available tickets in a city that has less than 50,000 people in the city proper when students are subtracted, is a rather daft move.
Put yourself in their shoes: for a decade, damn near every show was at capacity; and, those that weren't were easily at 80% or more (i.e., Pittsburgh isn't yet sold out for this show, but, after doing some math on Ticketmaster, I have determined that about 90% of tickets have been sold, sans whatever is available on sites like Stubhub). So imagine walking on stage in a city like Clemson, or Tampa, or the different cities in Germany that sold embarrassingly low, too. Not just for a band of U2's stature, but any band would feel an emphatic need to take a step back after that.
And, that's precisely what All That You Can't Leave Behind seems to be. Rather than feeling the need to constantly be "different", "edgy" and what not, they just wrote an overall collection of damn fine pop songs. To top it off, 9/11 happened in America shortly after the end of the European leg on that tour, with songs like Beautiful Day and Stuck In A Moment and Wakk On taking on new meaning.
The mojo that they felt they lost on PopMart, that had every reason to believe that they had more than reacquired it, as Bono famously quipped at the Grammys about "reapplying for the job" of being the biggest band in the world.
Fast forward a few years, and How To Dismantle An Atomic Bomb was another soaring success. If I'm not mistaken, it was also released close enough to the Christmas season for that to undoubtedly boost sales. The ensuing tour saw nightly capacity crowds, the same as with the Elevation Tour. This tour ended in Hawaii, the 50th US State, in a stadium. That sold out show gave them the confidence to again try and tour US stadiums, as we saw with their next tour.
But, we have the problem of No Line On The Horizon. It is a fact that the band had a slew of songs recorded with Rick Rubin, which they shelved to again had Eno and Lanois, holing themselves up in a riad in Morocco, in Fez, hoping to channel some of that Achtung Baby era inspiration. Sadly, the band seemingly became addicted to the Beautiful Day/Vertigo template of releasing a bombastic first single, which resulted in the misfire of Get On Your Boots. Also, it is a known fact that the band ditched the bulk of their actual experimental work from Fez to instead try and focus on writing radio hits.
But, the times were changing, as the digital music era, which U2 helped usher in with the Vertigo iPod commercial, meant that people don't consume music in the same fashion. YouTube was already established, as was Pandora; but, Spotify was just beginning to become the cult that it is today. The fact that Spotify can now be used as a verb today should tell you plenty about the status of that app.
So the elusive single was made elusive still more by this new climate. To top it off, NLOTH grossly undersold their previous releases to such a degree that many seem to be shocked by the runaway success of the 360° tour. In truth, NLOTH very likely sold MANY more copies than the 1 or 2 million worldwide figure that is generally touted. As digital sales by way of iTunes and Amazon was still a new thing (remember, both Radiohead and Nine Inch Nails had both done their digital releases just one year before, too), digital sales, at that time, were not counted in sales figures. Today they are, but in 2009 when NLOTH was released, they weren't.
So, in reality, that album like sold quite a bit more than the band realizes. The "sting" from its purported underwhelming sales is what ultimately resulted in the disastrous release of Songs Of Innocence such as they did.
I could keep going on, but I'm confident that I've more than made my point that I believe that U2 "gets it wrong" far more consistently than many may think, or may like to think. All the same, the 360° tour is both the highest attended and grossing tour; also, the Innocence & Experience tour was a rousing success, though obviously on a much smaller scale. These summer shows will likely gross more than that entire tour, despite its being roughly half the amount of shows. Also, I saw the I&E tour in Toronto, NYC, Belfast, and Dublin, and the audience, by and large, knew the new songs, even cheering them on when Bono would announce the song title.
In closing, if I could change the U2 timeline, it would either be with the music of NLOTH (I love some of the songs, but Get On Your Boots and Crazy Tonight are painfully obvious in their being radio ready, as well as Unknown Caller and Stand Up Comedy quite possibly having their worst lyrics ever, UK in particular, which is sad, as it has one of Edge's best ever guitar solos), or with their taking the official album sale count too seriously, or the release methodology of SoI.
But, what do I know? The tours still sold out! It's f****** U2 we're talking about, and people will buy their tickets almost no matter what!