There are many potential reasons why you might not be able to leave feedback, including, but not necessarily limited to:
Transaction was more than sixty days ago
Buyer or seller no longer registered
Item removed by eBay
Already left feedback
Comment contains profanity according to a bot
Forgot to check one of the positive/negative/neutral buttons
No text in the comment field
Lost a dispute
Dispute started and item no longer appears in Purchase History
Purchase made as a guest or using another account
Trying to leave a negative for a Powerseller without waiting seven days
Various hardware/software/network issues
If none of those seem to apply, it might be something else.
