I was having that problem too, but I found the fix for mine was to shut off the tax rates in PayPal since it was trying retax already taxed items, that was causing a mismatch in the amount and not returning the data to zencart.

To find the info on the missing orders, I downloaded and installed the
PayPal Sessions Viewer contribution. It's a bit cryptic, but it allowed me to find the info I needed and process my orders.

I have'nt had any problems since shutting off the tax rates in PayPal.

I hope this helps and works for you.