I've just had a similar experience. Though the kitchen gray tank has been dumped at least a couple of times in the several weeks we've been able to use our 310GK, it has never been dumped by *me* until a few days ago. When I pulled the handle, nothing at all came out after a week of liberally using the kitchen sink. When I left it open for a while, a small trickle began to appear.

On the advice of a mobile tech I filled the kitchen gray tank until it was gurgling in the drain (I also added a bunch of water softener and Dawn dish soap). When I dumped that, I got the gush I should get. The blockage it likely had has been displaced. For now.