Seems you have a problem with biscuit size, or your plate joiner's depth
of
cut ... you need to figure out which because this gap business shouldn't
be
happening.

I have done a thousand biscuits (PC joiner/PC Biscuit) with no problem, but
they were all 90 degrees. I think the clamping was adequate to bring them
together. Since clamping is at an angle to the biscuit on 45s, it just
isn't enough.
But making the cut a bit deeper for all biscuits would probably be prudent.