I would try this: Pull the spark plug out and see if it will turn over? A lot of gas may have leaked into the combustion chamber(where the piston goes up and down under the spark plug) it so this could cause it to hydro lock. But with the spark plug removed any liquid fuel will be forced out the spark plug hole, you will see a it being pushed out either in raw liquid form or heavy mist for several pulls. If it don't turn over with the spark plug removed something caused it to lock up.

IF you find that fuel is spurting out the spark plug hole then the dealer who cleaned the carb. need to do it over cause now the float is stuck open IMO.

You must be letting it sit with cylinder to the side or up side down. Oil sip into the combussion chamber and prevent the engine from turning. Take the plug out, pour anything inside out, and put the plug back and try it again. It will smoke a lot, might be hard to start. It might want to die, but keep the throttle and smoke will clear up after a minute or so.

GX31 can run in all position but it cannot be store in position where cylinder is lying on the side. The only position of safe storage is engine flywheel point vertical down or with cylinder point up. Or else oil will sip pass the compression and oil ring into the combussion chamber. When engine is runing, the compression ring push onto the cylinder wall due to combussion gas pushing from the back and make a good seal. When the engine stop, no more pressure from combussion gas pushing behind the rings and oil can sip through if it sit on the piston.