Thinking of huge 'Stadium' bands that are big in America but barely sell out a Theatre in the UK. Similarly big UK bands who are not fully understood in the USA. For example, KISS were huge in the States but never quite did the business in the UK. (I am a fan and I have noticed). List them...
Hootie and the Blowfish Dave Matthews Band Young Rascals/Rascals (no serious attempt made) REO Speedwagon (largely) Three Dog Night (largely) Grand Funk Railroad
Journey were largely unknown in the UK before Don't Stop Believing reappeared due to The Sopranos and Glee
Tommy James and the Shondells were a one-hit wonder in the UK ("Mony Mony"). There are a lot more from the other direction - UK artists who didn't make it big in the States. For example, glam and 1970s punk had little commercial success in the US; artists from that era are better known in retrospect than they were at the time.
American acts which didn't mean as much in the UK that spring to mind: Toto, Hall & Oates, REO Speedwagon, Survivor
I'm still very "So...?" about the continued popularity of the Grateful Dead. I just don't get them and neither do any of my close friends. Did nothing here. Going the other way, I've never understood why the American rock audience didn't take to the classic Quo albums, such as 'Hello!' and 'On The Level' etc. So it goes I guess.
The same in Germany, not many Grateful Dead-Fans here. Also no one likes Phish, perhaps they think it's prog.
I'm American and I'm totally mystified by it too. I don't get it. I work with a couple of Deadheads who play them for me and it does nothing for me. Zero. I guess you had to be there.
I saw them in 1980 at the Motor City Roller Rink in Detroit. There were no more than 200 people there. I'm sure they were playing to thousands in the UK at the time.
The Kinks were much bigger in the US than the UK in the 1970's. Pretty much the opposite of the 60's.
US acts whose popularity didn't cross over the UK (maybe?)... Billy Squier Rick Springfield Bob Segar The Cars
Beautiful South. But I'm not sure it's because Americans failed to "understand" them. We just didn't like them.
Squeeze headlined Madison Square Garden in 1987 so there were markets in the US that they were popular in.