The inability to achieve normal engine power output (e.g., poor static power) can be an insidious, difficult-to-troubleshoot condition that ultimately affects almost every small engine sooner or later. Sometimes, the cause can be pinpointed quickly, especially via multi-probe engine analyzers. But many times it can’t. The best bet is often analyzing all the power instruments that you do have, not just one.
It is more frequently encountered in engines that have not reached TBO but have weeks to months of sitting idle between flights with sometimes as few as 20 hours a year or less of operation. Also, flying past TBO may increase the occurrence of this condition. Static power is relatively easy to check with a fixed-pitch prop at standard atmospheric conditions. It should be done at every annual.