Results depends on how the recording was made at the studio.
If you keep doing what you're doing, and you are still hearing an "echo", the singer's track either may not have been recorded stereo "center panned", or even if it was, you may have removed the lead, but are hearing reverbs that were added in the studio, and are not center panned.
Either situation will produce the results you describe.
MTU is the only company I've seen so far that is honest enough to promote their product as a vocal reduction/removal tool, because in these instances complete removal just doesn't happen, and on many mono recordings, there's hardly any noticable difference to the average human ear.
Hope this helps preserve your sanity
George