Brian,
It must pass an rpm check before initiating the start sequence. It first engages the starter, then looks for an rpm reading of 5000 or more. If you are only getting up to 4800 you aren't passing the test and it won't continue with the start sequence.

If you push the "AIR" button on your start box what is the max rpm you can get? Does it make any unusual noise when the starter is engaged? If it is making a strange noise the starter o-ring could be slipping or you have some oil or dirt on the o-ring. If that is the case inspect the o-ring to see if it is ditry or needs replacing. Clean the o-ring with alcohol. If it is a bit worn you could flip it over and reinstall it. Put a drop of CA in the o-ring grove to keep it from slipping.

Let me know how you make out.

Joe