Probably not enough airflow from the relief ?
Or go back to square one and make a better box ?
I am only running two motors, but if I add two more.....wellll I think I could hold anything down.
I built my box from drawings off the forum. I didn't try to modify it in any way (except for cover 2 empty ports cause I didn't get 2 more motors yet)
So far it has exceeded my expectations! two lighthouse motors, some plywood, pvc......etc.
In the end just do a little research and don't reinvent the wheel?
Dave