The air flow adjusting valves may not look like an area for skimmer troubleshooting, but they can become clogged very easily. Salt is the usual culprit, at least fro my experience, and a blocked valve will restrict air flow to the protein skimmer. If the air valves are not cleaned every few days, they build up can eventually render your skimmer useless from a filtration standpoint. When this happens, the skimmer may run a little bit louder and the bubbles will be extremely fine. Additionally, the transition from water to air that occurs the skimmer’s neck, will be driven upward, far into the collection cup. Unfortunately though, the pump won’t have enough power to push the dried foam into the collection cup, and it will just accumulate in the neck. If this happens frequently or the skimmate is allowed to collect in the neck of the skimmer instead of the collection cup, the hobbyist will have to clean the skimmer more often.