when you crank down hard six things can cause your chain getting loose again:
1. shackles of your chain wear out when old, or "set in" when new.
2.backwheel moves.
3.you dig your chain into the aluminium of your sprocket (alu is pretty soft)
4.there's some play between axle and sprocket
5.bearings wear out and the material that cradles the bearings gives in (alu is pretty soft)(there are bearings in the bottom bracket and two undersized bearings rolling on top of each other in your freecoaster so that's a lot variables to take into account)
6.due to a worn out chain, improper alignment of bottombracket or backwheel, worn out or bent sprocket there can be a loose spot in your drive train. You could have tightened your wheel on the spot where it's the tightest.