>"You don't need to do anything too fancy to record vocals, but the mic should be well away from any walls, and the area directly behind the singer should be non-reflective. This could be an area of foam tiles or it could be a duvet, but one point to watch out for is that, in rooms where a lot of damping material has been applied, you'll often find that it only absorbs effectively down to around 250-300Hz. So what actually happens is that frequencies below 300Hz are allowed to predominate, making the sound seem congested or boxy."<
So glad they told you what the problem is, but how about a solution.
Really in a voc booth it is best to use 2" rigid fiberglass spaced of the wall 2". It might be best to put a few 4" pieces in there spaced 4" if you can fit it.. Don't forget about the ceiling.
Glenn