You could rule out issues with your account by testing another browser and seeing if you remain logged in. You could try clearing all data from all sites, in case there's something interfering with our cookies.

Other than those, could you give me a bit more detail on the issue. I assume you aren't seeing this problem with other sites? Does the site log you out after 15 mins or so, or is it after a longer timeframe that you are logged out, maybe after a browser restart?