Make sure you are signed in to the same eBay account that the messages are received on. Or try replying directly to the email rather than using the reply button in the message.
Some general advice if you are having problems viewing messages:
If you are using a mobile browser, try using the eBay app or else request the desktop site version of the page with your mobile browser. Or try another browser.
Details about requesting the desktop site version of the page in various browsers can be found here:
https://community.ebay.com/t5/Technical-Issues/Will-not-stay-in-Classic-Site-mode-listings-appear-in...
If you are already using a desktop browser, try a different link such as this link to messages from members:
https://mesg.ebay.com/mesgweb/ViewMessages/0/m2m
If all else fails, try reading your messages on ebay.ca instead of ebay.com.
You should be able to go to a user's feedback or profile page and compose a new message there using the "Contact" link if you are unable to respond to a message. If prompted for an item number, try selecting "not about an item" if the item number is old or you get an error message.
