Very weird reverb problem with dspMixFx

Hi, Long story short, while tracking to record a vocal the backing track is swamped in reverb from the dspMixFx, it should just be on the vocal, I have a different project where this doesn’t happen and the settings are the same in the dspMixFx window…just cannot figure out what’s going wrong.
any help much apreciated cause I’m going nuts.
also should the fx bus connections be used…what are they for? ok they are for the FX I hear you shout…but under what circumstances would I use them please.