Take out the Filter cartridge...does it spin normally? If so, then install a new one to see if that helps eliminate a flow problem internally.
Also could be an issue with the intake tube not pulling enough flow due to a clog or something. Like TabMorte had with his, you may not even need the extension depending on how deep your tank is.
If taking out the filter cartridge doesn't help, then LMarts is prob correct.//one of those bearing may not be seated properly, or has a tooth damaged, clogged, etc.