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
Purchase made as a guest or using another account
Trying to leave a negative or neutral for a Powerseller without waiting seven days
Various hardware/software/network issues
If none of those seem to apply, it might be something else.
Try using this link; see if the transaction appears and if you can leave feedback:
https://my.ebay.com/ws/eBayISAPI.dll?MyeBay&&CurrentPage=MyeBayFeedback&ssPageName=STRK%3AME%3AMAFBX...
