It is an easy matter to determine if there is a leak in the overflow box. Turn off your return pump, allow the overflow to drain, and observe to see if there is significant amounts of water entering the overflow box in places other than the top. If not, then I would suspect that your return pump is either inadequate or has been clogged or has something wedged in some way to reduce the amount of total flow reaching the tank. Those are the only explanations I can think of to account for what you are seeing.