Volume:
If the fault can not be reproduced on ground (e.g. due to a temperature related problem, like thermal stress on a cracked solder joint) the units are declared "serviceable" and end in an aircraft again. Just when the frequency of one specific fault indicates a real issue which can be pinpointed to one specific system, more action is taken.
The classic, decades-old approach to troubleshooting such problems is to swap components between systems (L vs R engine, PCU#1 vs PCU#2, etc.) or between aircraft (Nxxx vs Nyyy). If the complaint follows the component, then the fault is isolated. If not, keep looking.
This method often works when others fails, e.g. on intermittent problems. Would Air Asia Indonesia have this depth of understanding?