I have the same issues, and have been working like mad on it.
I've just checked my grip, I use the following and it works for me, full use of thumb and index finger, and the tips of my middle and third finger, with a very relaxed grip, I find the power i need in drives can be generated without gripping too hard.
My coach had me playing with just the top hand last week for 10 mins:(
If you fully grip with your bottom hand, the mechanics of the joint make it difficult to play properly.
Good luck, and just try things until you are comfortable.
If you are netting you may get a whole heap of different deliveries, so I would use the opportunity to get some proper throw downs or ask the guys to pitch it up for you.