I had two problems with PayPal. One was with the USPS shipping that had to do with getting active credentials from USPS as opposed to the testing ones. This issue was solved in this thread (The WebAssist official instructions are incorrect): showthread.php?p=48379#post48379
And the other was with the Sandbox API not changing out. (Even though I had changed it in my store's settings.) The Sandbox problem was solved here: showthread.php?t=12382
Hope this helps you!