If you are referring to the manufacturer “numatics” and their lap fit spools it could be the type of lubrication you are using. I came across a problem with a lap fitted spool valve where the customer was using hydraulic oil in his lubricator. The valves would appear to stop shifting but actually only shifted very slowly. Almost like they had a retarder, no matter hooow hard you pushed the spool by hand its movement remained very slow. A light push got the same results as a heavy force. Clean them out with solvent and they would operate for another period until enough oil got to them. Naturally the customer thought the fix was more lubrication so the problem continued to get worse instead of better. I never found out what was in the hydraulic oil that caused the problem but after its use was discontinued the problem ceased. I don’t remember what oil they started using but I believe it was a non-detergent 10 Wt. engine oil. Just a thought.