Singers who were big with a band or duo then struggled solo

I got the impression David Lee Roth's first two solo albums did well. Then he fell off the map.

Roger Daltrey's solo career met with a fraction of the success he had with The Who.

Paul Di'Anno from Iron Maiden
Geoff Tate from Queensrÿche
Joey Belladonna from Anthrax
Tony Martin from Black Sabbath
David Vincent from Morbid Angel

Bryan Ferry/ Roxy Music
Donald Fagen/ Steely Dan
Ric Ocasek/ The Cars
Jane Wiedlin/ The Go-Go's

Izzy Stradlin and the Juju Hounds

Darren Hayes -Savage Garde
Rob Thomas-MatchBox 20

I'm surprised nobody has mentioned Steve Perry yet. Even the filipino guy is doing better as a karaoke singer than Steve ever did with "Oh Sherry"
Also, you could consider Robert Plant to be not as successful being solo than with LZ.
Oh and Sebastian Bach.