Subscriptions - not activating on public side

Hi - hoping that I'm missing out a basic step. Here's my problem.
I set subscriptions and gateways - activated them and made them public.
Tested them and they worked.
Then, went back to change the subscriptions in the same way. They appear on the public side as an option but without that small box that seems to appear by the Paypal button - and when you click them it doesn't go through but returns to the register page.
Have I missed out something?
Ben ( Thanks