Black Tank valve failure scenarios

Thanks all, for the responses. I asked because I had trouble emptying my black tank recently. Luckily, no one was in-line waiting. Kept filling up a gallon container and dumping it in. In and out of the trailer seeming forever. Finally cleaned it out. Felt like the black tank valve was not as smooth feeling as the grey. I thought from the feel might some tissue bunched up at the valve in the black.
Anyway, much appreciated.