Without hearing you, it is difficult to know what your problem is, or even if you have a problem.

You should ask the people who've told you you sound better alone to tell you what is different--Is it an issue of volume, or are you not together on the beats, is there a "pitch problem", is there a conflict in styles, or is it simple that the fullness and character of your voice doesn't come through with a band.

Each of these has a different solution.   

The worst case scenario, which people are hinting around, but haven't spelled out, is that you've learned to sing unaccompanied, and have to learn some basic musical skills in order to follow a band or accompanist.

This is a problem because it means that you have to learn a lot of stuff, particularly related to following a beat, counting and phrasing, and similar things, and to unlearn a lot of habits, before you are going to be able to work well with a band.

The thing is, there is no way for us to tell which situation is the one that you have to deal with.

A simple place to start would be to try singing along with a metronome, to see if you can follow the beat.