They may have stored the rifle without cleaning and closed the bolt. This will carbon seize after a while on the gas piston and op rod. I always store my minis bolt open with an empty mag. You may have to disassemble and spray a cleaner like brake cleaner, around the gas piston to free up the action. Also check to see if the guide rod was assembled upside down, this will make it hard to operate as well. The tang on the guide rod should be on top of the holding pin, closest to the barrel.