After a user finishes their transaction with Paypal, they should be returned back to your website. If they are not, their subscription status will remain as "Pending." The entire process needs to be complete, for the status to be updated.

UBBT subscription page @ your forum
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Paypal payment @ paypal dot com
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UBBT subscription confirmation @ your forum

If the user stops at the second step, or closes the browser window, or sits there and lets the Paypal page time-out, then they have never returned to your forum to send it a "success" update. And your forum still sees the process as "pending."