Air Venturi Air Venturi Rove Air 4500

[flyfisher]

I've had the compressor for almost 2 years now and am using to fill a Barra 1100z first gen. I generally shoot my air rifles during the colder winter months but decided to start early this year. I'm filling to about 2,500psi. Now for the problem. I shoot till the gauge is just inside of the green ( 20 pellets) I top it off rather than take the gun down to 0psi.What's happening is that the compressor gauge needle stops moving around half way, the compressor sounds like it is running normal under load. If I use the air relief valve to remove air in the fill tube but leave the compressor running then close the valve. It will fill all the way to stop. I hope I haven't overly complicated my problem but am curious as to what might cause this.
Thanks Bill