PayPal Payments Problem
 

I have my subscriptions set up using PayPal and it works great when it works. But I have had a few users who haven't gotten past the "Order Using PayPal" button. When they click on it, instead of being taken to the invoice/purchase page some users are being sent to PayPal's homepage with none of the subscription info and pricing being transfered. Any idea what the problem is? So far one user on a Mac running Firefox, and a Vista user running Explorer 7 have been confirmed as running into this problem. About 70% of my subscribers are having this problem and I've had to set them up manually but I can't confirm what software they are running to learn if there is a common link.

Anyone had problems like this? Any help appreciated.