EC-130 Maintenance Inspector reported finding an NR sensor discrepancy due to the shims not installed properly.

Narrative

Day 0 of the NR (rotor RPM) sensor install (Reference; Airbus EC130T2 AMM (Aircraft Maintenance Manual) 62-31-00; 4-3; Page 13; F.18.) Fatigue: Mechanics and inspectors had been working a lot of overtime; including weekends; for several months. Distraction: A lot of mechanics and avionics technicians were working on aircraft SN: XXXX; there was constant commotion. Pressure: Due to the expected delivery time of the aircraft; there was a lot of pressure to get the aircraft into flying condition. Stress: Due to the time constriction and the workload everybody seemed to be stressed out. Person A and Person B were working on the assembly of the main rotor mast (that includes the NR sensor). Person C; a new inspector; was doing the inspections on Aircraft X at the time. I was not involved with the installation of the NR Sensor.Day 1; I returned Aircraft X back to service. I was not aware of the discrepancy of the improper shimming at the time. I also returned the aircraft back to service on Day 3; Day 6 and Day 7 and I was still not aware of the discrepancy at these times.Day 16; I was made aware of the discrepancy by my supervisor Person DSuggestions: Maintenance: When performing maintenance on any part of the aircraft and the maintenance manual is calling out for a critical measurement; that measurement needs to be performed. Even if the part was removed from the aircraft with that measurement in place. A mechanic and inspector should at least confirm the validity of the measurement if applicable.Fatigue/Pressure/Stress: There has actually been talk about a 4 day/10 hour schedule including a weekend 3 day/12 hour filler schedule; which might solve this issue. Alternatively; the mechanics and inspectors could go through a NO-OVERTIME and NO-WEEKEND-WORK schedule during very busy times. Split the work force in two and make it a two-week rotation schedule: Allowing every individual to have a full weekend every other week and leave on time one week and work overtime the second week. That way work could progress; while individuals get time off to decompress and unwind.Distraction: Go three steps back in your maintenance procedure before continuing the work.