It isn't a big mystery. In both cases you are nearing the limits of the compressor.
Compressors are rated at a specific PSI at a specific flow volume in CFM. In other words they can provide the rated PSI up to the rated flow volume. Above the rated flow volume the PSI will start to drop, sometimes dramatically and sometimes more gradually. If you hook a garden hose to your home water supply you will get pretty good pressure when you open the valve. If you hook a fire hose to your home water supply you are only going to get a small amount of water coming out at a relatively low pressure. Same situation with air compressors.
I have no doubt that both of you are using compressors without a tank. A tank will eliminate that problem since it effectively isolates the compressor from the load. Small compressors may take a very long time to pressurize even a small tank, however, and the demand may empty the tank much faster than the compressor can repressurize it so a tank won't solve all problems.
The bottom line is that your compressor is doing all it can do and there ain't no more.