I have a 2008 Select Suite and had the same problem plus it did not hold water. I ended up replacing the valve and did it myself. If possible disconnect the cable from the valve and see if the cable operates freely. If so then the valve may need to be replaced. CW and other RV supply places sell replacement valves but they come with a 6 foot cable, mine and possibly yours is a 10 ft cable. Do not pull it out because it is a bear to replace it back into position. When I replaced it I used lots of vaseline on the gaskets and the sliding part of the valve and have not had any trouble since. B-T-W, nothing came out and dripped all over the place when I removed the old valve because I did a thorough backwash before I started.

In my opinion the long cable is most of the problem. In my 07 Elite the handle was almost impossible to move and never sealed the black tank completely. In my 11 Escalade the black tank valve is stuck open and inoperable. The warranty fix was to add an aftermarket blade valve at the end of the sewer outlet line. (no cable involved) It works great but when its below 32 I leave it open as there is about 8 ft of sewage filled pipe under the heated part of the trailer that can freeze.