GA160 Atlas Copco Screw Compressor

We constantly have an issue with loading the compressor. Electrically everything checks out. there is oil in the loading and unloading shuttle. We disassemble the shuttle/valve and lube it with white lithium grease and it works perfect for several weeks then it sticks and will not load. It appears the oil cleans the grease off the valve and then sticks again. Should there be oil in the valve\shuttle? Any ideas?