Sounds like the (2) channels are not "linked" in a stereo configuration (as opposed to a "dual-mono" config). This is a standard feature in most 2-channel hardware compressors to avoid what you are complaining about. Try that...

It can also happen like this too:

You set a compressor for a fast attack and slow release...the bass that is slightly off to the left disappears because the attack is too quick for it so now the left side seems off balance and the right becomes more prevalent...etc.

This is just an example of one scenario - perhaps the easiest to predict. However, bass isn't usually off to one side, so this is probably not the exact cause, but it could be that squeezing too hard, too fast, too slow or a combination of all of the above is too much for it.

Do you recommend I use a stereo compressor then on bass and vocals even though they are mono?

Also I'm using the channel strip which is multi band, maybe the frequencies are pulling each other?

Thanks for your tips'

Mono on mono.
Avoid the multi-band like the plague.

Maybe EQ, then compress?

Maybe EQ, then compress?

Sometimes, yes...

Other times, no...

Which sounds better?

Sometimes you feel like a nut, sometimes you don't.