I had this issue a while ago. What I did was to turn off the water. I then used a piece of coat hanger and bent a small "L" into the end and used the "L" to gently scrape around the groove in the seal to dislodge any foreign material that was stuck there. Then I sprayed a pure silicon lubricant into the groove and over the slider. It's worked for our leaky toilet slide valve, so far.
Of course, maybe you need to replace the flush valve, but maybe not.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch