APU Failure and Low Pressure

Last night as I sat at the gate in Copenhagen getting ready for a quick flight to Oslo I kept getting APU "low pressure" light followed by "failure" light. No biggie, I read the QRH and it said shut it off, wait five min and if the light goes off (it did), try a restart. Did that and had same failure. So I went into the FMC and cleared the failure, did the maintenance on it but it still would do the same thing no matter how many times I cleared the failures via the FMC. Question is why? Can you not clear failures with any systems on? I have about 20 hours in the NGX and never seen this before where a failure once cleared via the FMC would continuously pop back up.