There are many potential reasons why a user 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.
Without knowing exactly what your buyer is doing and seeing, it is difficult to provide specific advice, beyond the usual: try another browser, another link, another device, another site (such as ebay.ca instead of ebay.com) to leave feedback. Or try again later.
