Question: Is the drain handle pretty easy to pull to open/push to close? There are four bolts around the gate valve. These apply pressure to keep the o-rings from leaking. I've noticed that if those bolts get loose the gate valve slides easier.
Either way the first thing I would do is open up the belly so I could check the valve area for your leak. If the leak is there it may be as simple as snugging up the bolts. Or maybe replacing the o-ring.