Head holding tank not pumping out when being pumped at pumpout station

[Capt Mark1]

Not sure what the answer will be on this but I'm open. I had black water coming up the tiolet. I would pump it down and then it will come back up. I pumped the system when off shore but it didn't pump all the way out. it resolved the problem short term. I went to the marina pumpout station and put the hose on and nothing was coming out, not even a drop of water. (waste). I can see that there's waste in the tank but nothing was pulling up. OK now what the heck is going on here. Am I missing something ? Any Ideas ????