|Here is the obvious question: could it be buildup? I don't have much experience with leaks, but buildup is one of the usual suspects, so if you haven't already, do a hot wash with no soap and verify if there are lots of bubbles when the machine is agitating. If there is, strip your diapers by washing them a few more times in water as hot as possible, with no soap and maybe some vinegar to help dissolve the buildup soap.
If not, do you use fleece liners (I think buildup can also occur in those)? Or could it be that it's time to go up one size in covers? Often, even if the cover doesn't seem tight, it can help to go up one size. I assume you make sure that no clothing is creeping out and that the diaper is not creeping out of the cover...
I don't have any other idea, but I'm sure other people will help.