What is not functioning in the head?
If the seacock is closed you will not be able to push down on the plunger while it is in the WET position. It will evacuate the bowl in the dry position just fine.
If the seacock is open and you still can't push the plunger down in the 'wet' position, then there is a blockage between the intake and the head.
It the seacock is open and you can pump in the wet position by no water fills the bowl, then the pump needs a rebuild.
If the pump is in the dry position and does not evacuate the bowl, then the pump needs a rebuild.
If the pump is in the dry position and you still cannot depress the plunger, then there is a blockage between the head and the holding tank, or the holding tank vent is blocked.
What problems are you having? Remember, we're not charging by the word, so feel free to use lots of them LOL