If it's not your vent, you may have accumulated solids in your tank that need some help, er...being dislodged. A black tank flush wand connected to a garden hose (not your drinking water hose, of course) is what we used on our black tank when we had our TT b/c we didn't have a built in tank flusher. We used it after we dumped the black water tank.
How to Use A Flush Wand. I also moved the wand up and down a little (careful doing this!), as well as side-to-side. You can get one at any RV store. This is best done at a full hook up campsite, as people will not be happy waiting for you to do it at a public dump site.
In case you didn't already know this, always keep the black tank valves CLOSED while camping until dumping to avoid the dreaded pyramid effect of solids, which then harden like concrete. We also kept the gray tank valve closed the last day of a trip to be able to use the gray water to help flush the sewer hose out after dumping the black tank. Keep the grey valve closed and the black valve open when flushing. After several minutes*, close the black tank valve and open the gray tank valve to flush out the sewer hose before detaching it from the camper.
*You can buy a clear connector (attaches to the camper on one end and your sewer hose on the other), so you can see when nothing more is coming out when flushing. TMI, I know...
Some people swear by this: The next time you go out, put a couple of large bags of ice (or several small bags) in the empty black tank, then drive to your campground and empty it when you get there. The ice helps dislodge solid matter that may have built up in the tank.
