Trouble description: Main air compressor taking longer to fill up Main air bottle compared to other

Trouble description: Main air compressor taking longer to fill up Main air bottle compared to other compressor.

Parameters (Normal/Present): Discharge pressure at each stage

1. 1st stage - 2.5 bar,
2. 2nd stage - 9 bar,
3. 3rd stage - 30 bar.

Present Status: Checking for external leaks. No major leaks found. What could be the reason for slow filling rate.