TPMS

I'm having trouble with inflating my tires, and checking the tire pressure....the pressure is around "27-29", if the pressure gauge is accurate, but it keeps giving me a slightly different # each time I test them and it is the same when I borrowed my neighbour's two pressure monitors. The tires are nitrogen filled, and the TPMS went on the other night so I checked all the tires. I noticed on two of the tires the valve stem has a red rubber part inside, and on the other two it doesn't have that...don't understand why that is. I have tried to inflate the tires that are lower, but they won't accept air and the pressure gauge goes beserk way past the '35psi' which is apparently the recommended value for this car. What am I doing wrong, or have I got defective valves?

Mystery solved...the little red rubber thingys at the top of the Nitrogen valve covers had somehow gotten inside the valves and plugged them. This gave a slow leak in one of the tires and plugged the valves so they couldn't receive any air. Apparently this was something that should have been checked at the sale of the car and wasn't. The dealership removed the embedded rubber out of the valves and filled up all the tires with nitrogen for free...problem fixed.