My problem is similar to others I've seen mentioned, but I'm not sure how to fix it.
On forms that have been edited, paypal doesn't work. It goes straight to a screen saying the registration form has been received, but the payment screen does not appear. Following advice I've read here, I tried deleting the associated paypal form and making a new one, linking it to the edited form. This doesn't work.
An example is here:
http://pbgfc.com/tournaments/international-billfish/international-tournament-registration/
I will send login details privately if you need them.