Cam Sprocket wont fit. Pit bike

[Bansh88]

Got a new piston for my daughter's 125 SSR. Previous ring blew out on the piston.
Assembly going good until time to place the cam sprocket. It's about 1/8 -1/4" short. Pulled head a few times to make sure chain wasn't hung up. Chain roller seems fine.
Turning the magneto feels smooth, no hangup or chirps.
Any ideas? Internet seemed to show no similar problem...

 

[Kraut_n_Rice]

How's it tensioned?

 

[Jersey_Jeff73]

probably have to release the chain tensioner