I have 4 showers in my house 1 downstairs and 3 upstairs. Two of the 3 upstairs have stopped having hotwater in them, shower A is a tub shower combined unit and has no hot water in it, shower B is a shower only with a jetted tub in the same bathroom but not single unit. The shower in B has no hot water but the tub is fine?

I'm searching for your "venting issue" and haven't found it yet. But your water issues put the problem squarely on the valves. Sounds like they're loaded up with mineral buildup caused by not flushing your water4 heater on a regular maintenance schedule,( more about that later).
Open up the valves in question and check and clear the inlet ports on the cartridges. Before you reassemble, turn the water back on to flush out any crud remaining in the supplies and valve body.
Now, back to the water heater. For long life and fewer troubles you should keep your heater clear of mineral build-up by flushing on a regular schedule. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white or yellow grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear. Now flush out your hot water lines on ALL fixtures that are affected . Now pull each aerator and clean the screens. Be sure you put them back togather the same way you took them out. Don't forget to flush it out every month. Your heater will thank you for it. Hope this helps, Tom