Are you using an iPhone? If so, are you using Safari? It seems to be pretty picky and might not always log you back in, depending on how long it's been since you visited, from what I've been reading on the web. If this is the case you might try Chrome if you haven't already done so.
Also I made a change to the forum so that it will always redirect to https (secure connection). It wasn't doing this before (though I thought it was) and this can cause cookie / login problems.
Hope it works better for you, if you are still having any issues let me know!