Biggest problems for our girls was fear. Fear of hurting the bases, not getting high enough, not getting to the right point,...
That led to them being very careful and thus not creating a good rebound. They also tended to pull their knees up in order to make sure they reached the bases hands, which of course cut off the remaining rebound completely.
Flyers basically have to imagine that there are no bases waiting for them and just go for their tumbling pass...
Bases in turn have to react quickly to where the flyer is heading and then make sure they don't lean/step out for fear of getting hit.