I am confused!. I have 2 pressure tanks in series. Last week I went to check the cut in pressure. I cut off the pump and drained the tanks. The second tank in series was about 10 # low but the first tank would not register on my gauge. When I tried to put air in it, it would not take any air. Naturally I thought the core in the stem was bad so I replaced irt.,. It still would not register nor take any air. The other tank is working just fine. Even if the diaphram was busted it still should take in air,, so what could be the problem be with this 1st tank?

Did you try pulling the core and seeing if it would at least allow air in with no core? If it won't, then you will need to carefully probe into the opening with a small shank screwdriver (or whatever) to see if something has obstructed it.