When I used to do communal breeding which I was very successful at the females would stay in the same tub at all time (the male was the one moving as he would rotate between tubs)

If the females were not together prior to this it could have been stressful and lead to this

It can also be because the tub was over crowded

One of the moms might not be a good mom.

The babies might not have made it and they cleaned up.

Can be many things.

Since you had that issue I would encourage you to separate the females next time and let them birth individually and observe them, this will tell you what kind of mother they are, if you have a bad mother you will need to remove her from your colony.

I would also encourage you to keep the females together at all time if you want to do communal birthing and avoid introducing females to each other during that time.