Scroll down your list of transactions until you notice anything different. In my case it was a four month old purchase that was lacking the little grey circle containing an arrow (grey for purchases, orange for receipts).
In my case the seller had not had part of an order in stock so they sent part, split up my payment and billed paypal in two parts. After they billed the second part, they never marked the original payment as "completed" even though both parts were completed and everything was received.
After locating the item causing my card to be locked, it was easy for the paypal representative to look at it, mark it as completed, after which I was able to delete my credit card.
... so try to think of a purchase in which the vendor might have split up your original payment and billed parts separately. I have had that done by Sears, Swanson, Lowes, and I think by Amazon.
I cleared mine by talking to paypal but you could probably have it cleared by calling the vendor if that is easier for you.
good luck
