First off, this is the scenario:
8x8x16 room, good vapor barrier.Room only opened 2-4 days per week,and 4-8 times on those days.
21C +/-1C
35% +/- 3%
Condensing unit: MMO12E-PA50A
Air Handler: PEH014A-P3L
Precision weighing lab with 20-23C, 30-40% requirements.
Unit has been able to keep these conditions over the past 4 years with very few exceptions: high ambient temp days, refrigerant leak, etc.

Recently over the past few months the unit has not been able to pull humidity out of the room below about 36% and currently has difficulty getting much below 40%. In the past I could set the unit to 30% or less as a test and it could handle the task. Now, no matter what humidity point I set (that is below 35%) the room will not get below 36-37% humidity. The temperature control has been working fine keeping the temp at 21.7C +/- 0.5C.

I have had an HVAC service tech come out to troubleshoot only to tell me that nothing is wrong with the unit. I know that there IS something wrong because the unit will not pull out any more humidity out of the room. I just don't know what the problem is. I would appreciate any help with identifying a solution to the problem.