That's going to be a "you" problem. I assume you're using a browser (Chrome, Safari, other?) on your phone. Are you selecting the "Stay Logged In" box when you log in? Are you regularly deleting cookies on your browser, or don't have them enabled at all?
Make sure your browser is set to accept cookies from TOO. Some browsers have settings to delete cookies once the browser is closed. If the browser is in private mode it will not accept cookies either. Some browsers like duckduckgo will only save them if you tell them to and they will only ask you once. Let memknkw what browser and platform and we'll get it figured out.
Yeah that would be why. You can do private browsing but allow specific sites to keep cookies. It's somewhere in the settings. What browser are you using? Maybe try clicking the URL in the browser and seeing whats there.