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 in this thread on the US discussion board:
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 browser or a different link such as this link to messages from members:
https://mesg.ebay.ca/mesgweb/ViewMessages/0/m2m
If all else fails, try reading your messages on ebay.com instead of ebay.ca.