My freind was having the same trouble with his RB 12 and the 1/10 scale box this weekend. it's not the compresion but the "fit".
Is it tight at the top even without a glow plug in?.
If so this should losen up after a few runs

I had simalar trouble when I used to race quickie 500 with Nelson engines. When they were new you could hardly turn them over

You'll need to set your box up so the switch can get the motor spinning full speed before the flywheel comes down on it. and make sure the piston is not locked up when you try to start it
Of couse make sure your battery is fully charged

Are you using the stand offs so the chassis is completely off the box?

After a while it should loosen up for you

Good luck