I just bought a birthing/gym ball to use in my last few weeks of pregnancy, and possibly in early labour.
I've used a good electric air compressor to inflate it, but the ball doesn't seem firm enough even though the air pump has kinda reached maximum capacity.
It is a bit squishy, which means it doesn't quite give the 'instability' you're looking for in a ball. Does that make sense?
So my dumb question is, uh, how did you inflate yours??