I don't know exactly what the problem could be, but I would suggest doing the following.
If you have batteries in your unit and they haven't been changed for awhile, I would go ahead and change them. I don't know for sure about the WK-3700, but a previous Casio I had would develop problems with old batteries in it, even if I was actually using the AC adapter.
Secondly, I would do a full reset of your unit. The instructions are in your user's manual. If you can't find them, then post back and I'll get the steps for you.
Don't know if this will help, but it's worth the few minutes it takes to try.