Humidity issue

[tipsrfine]

You might try and collect the condensate comming from the unit in question to determine whether the unit is removing the proper amount of moisture from the air. If it is, then you would obviously need to look into where the excessive amount of moisture is comming from-internally, externally or both. A pro here called Teddybear has a formula you can use that would be helpful.

[beenthere]

temp deltas should be taken at the air handler.

Your duct work has some leakage, no such thing as zero leakage.

[just_opinion]

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So, Since I've had the a/c company out twice, and they told me nothing was wrong, I bought a humidity gauge and started some testing of my own. On the two units that seem to be able to maintain a comfortable level of humidity, these are readings: supply temp of 10-16 degrees below return air temp and humidity level at supply of 30-40%. On the unit I feel is malfunctioning, I am getting a supply temp of 10-16 degrees below return air temp, but the humidity levels are 75-90%.
...

Thanks in advance for your help!

The humidity level is at the the grill where the air the blowing out ??????

If it is from the supplying grills then, the humidity readings for the 2 good units are very strange.
If not (which mean the air in the room/area), then the data for the large unit is very strange too.

Please clarify the data one more time.