Cleaning powerheads for me involves:
A) Removal of all powerheads from aquarium every 3 months.
B) Removal of impeller cover for impeller and impeller well cleaning.
C) If Powerhead is 9 months or more old and has an impeller shaft (ceramic or metal), it is replaced. Metal impeller shafts tend to get worn down over time.
D) Impellers are replaced every 12 months or earlier if needed. (Noticeable wear)
E) The impeller well and intakes/outflows are cleaned under the tap with a q-tip.
F) Any calcuim deposits which might have built up on the inside of the powerhead are scraped off.
Typically, if the powerhead is having a noticeably reduced flow, the impeller shaft is probably worn down, or there is a buildup of organic matter in the impeller well. If there is a powercut to a powerhead and it doesn't restart, chances are there is a buildup of organic matter in the impeller well. Giving the powerhead a good shake or tap will restart it, but that isn't recommended. If you notice a problem with the powerhead restarting, remove is from the tank and perform a cleaning (See above). Any organic matter or calcium deposits between the impeller and the motor will cause the impeller to not spin. They work on a magnetic field, and if the field is impeded, the powerhead won't work.
-Ayres