Front Airbags not filling at idle??

I do not have a schematic available for your system but I will make an educated guess. Somewhere in the system there is probably a one way/poppet valve to maintain pressure in the front suspension and another for the rear in case there is a failure of the pump, air storage tank or plumbing. You do not want the suspension collapsing unexpectedly at speed. The one for the front may be sticking shut and not allowing the front pressure to come up. You could also have a system that uses a rotary valve to feed the suspension systems separately and it has an internal poppet/valve to the front sticking intermittently.