Here are two thoughts:

Best bet is to try a different ECU battery, maybe from one of the other Jetcats that are working fine.  Possible that lipo or starter motor just need a couple break in cycles. I once had a new 20C 2S 4000mah lipo that would not start one of my engines while 3 others of the same type did.

Possible there is a little slippage of the o-ring on the shaft hub.  Usually you would hear it if it's slipping.  Apply a slight side force on the starter housing with a finger and see if that gets the engine to spin up better.