Primary air tank won't build air

[dodges1515]

Which air dryer assembly do you have? If it is a AD-IS it has a built in manifold that directs compressed air to different locations on truck and the holes are quite small,(like 1/4 inch)if your hose/line from compressor has internal issues of breaking down it can plug these holes.It is fairly easy to take apart for inspection.Seen this several times.
Also it really doesn't take much of a air leak along with a marginal compressor to get behind on any particular tank psi.
Bendix online has all the specs on what your compressor should perform at and also an exploded view of your dryer assembly.

Post your dryer type and also compressor type it will help a lot,many people on forum have lots of experience in this area that can help.

Good Luck!