If you remove the glow plug and the pull start still does not pull then yes the piston ring is locked inside the piston chamber and the piston head has swollen no need tp panic though this is very common on new nitro engines. First to avoid this is to allways let the engine cool off with the piston fully at the bottom of the motor the best thing to do is to remove the motor take the pull start and the back plate off of the motor you will see the piston and the piston rod. Take a hair dryer and let it blow into the motor to get the piston hot with the glow plug removed put some after run oil in the top of the motor. After the engine is hot turn the fly wheel with you hand after a while it will break free. Once you have it loose make sure to leave the piston at the bottom of the motor where you can almost see the top of the piston from the back of the motor. Put the backplate and the pull start back on and re install the motor and it should fire right back up. I have found that for all of my motors that start out this way always last loger and keep more compression then the one that I break in that does not pinch.