Three points:
1) If such a dynamic parameter as FF stays aboslutely constant, I would be suspsicious.
2) Talk to your pilots to see what is the normal value on the FF (for me 3,6 sounds like a TOGA on a hot day for 27,5k engine)
3) FF on the engines should be equal +/- wear and tear. Absolutely not possible to have 1 t difference!!
My best guess is that you have problem with the FAP profile definition.