Weights not updating if I re-do the flight plan?

[chkl 34]

I could be imaging things... and could be doing this wrong or missing something obvious... but lets say you create a flight, and put in weights that are too heavy for the flight level you want to fly. PFPX tells you "altitude above max" or some similar message. So you go back to the payload section and enter a lower payload and it should be light enough to be at the cruising altitude you requested, but the same message appears, "altitude above max" after the new weights are entered and you try to calculate the flight over again.

 

I am flying the PMDG 737-700 and the weights when put in the FMC show me I can fly to FL400 but entering a cruise of FL380 PFPX tells me that it cannot cruise at that altitude? Am I doing something wrong or is this a bug in the PFPX programming where it is not refreshing the loads?