Have you "adapted" the dip tube (the long "beer out" one)?
The unmodified dip tube reaches the bottom where all the yeast is! Many folk don't bother modifying, and just put up with the first few pints containing yeast as the area about the dip tube gets sucked clean (they may also finish fining, and perhaps even filter, before casking and "force" carbonating - not for the "real ale" brigade like me!). Some cut 30-40mm off the end of the dip tube. Some (like me) bung the end up with a piece of old O-ring off the lid and drill a hole (cobalt drill and a little "jig") about 30mm up from the bottom.