it probably is blocked and it could be a real pain to clear without replacing the nozzle in the tank. You could try fist using some compressed air to blow out any water in the line. Do not make a solid connection, just blow air into the opening and let the water in the line flow back out same. I would then try a tank sensor cleaner and tissue digester put directly into the line by pouring the liquid into a hose and then connect to the flush out connection. Push it into the line and turn off the pressure. Very slightly, loosen the connection to allow air to escape. Re-pressurize the line. Do this several times to be sure the solution has reached the nozzle. Let it sit for a few days. Open the dump valve and Try pushing it out with water pressure. If it does not clear, you could use air pressure up to about 100 psi as long as the tank has a way to vent the pressure once it blows free. Good luck and hopefully this will solve your problem. If not you will have to access the nozzle in the sidewall and replace.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7