Very Musty properties with some mold growth

[Scott Radetich]

Looking for some advice

I"m under contract on a few single level 1970's duplexes, but two units have that very musty air sensation in them.  You know the kind i'm talking about, you want into the room and in a few minutes you feel that tickle in the back of your throat. 

During the inspection, a look under the homes showed some failed vapor barrier's.   I figured ok, easy fix, put in a new barrier and the issue would be solved.  But it wasn't, I went back after it was installed and the air still bugged me.  The inspector said if there was more ventilation in the roof, the issue "should" be solved, because it appears the moist air is just hanging out in the attic.   A little mold was mostly concentrated where the walls meet the ceiling and it was right below where the soffit vents are (the vents were also very clogged).  We had mold tests done and the levels were ok, and there were no "bad molds" detected.

These are located in an area with higher water tables and each duplex has their own sump pump. This is on a street of like 10 nearly identical duplexes and the others are all occupied so i'm figuring this can be fixed.

So my question is, has anyone dealt with this and is adding more ventilation going to do it?  Is there anything else i'm missing that I need to think about in terms of this musty air?  Cause as of now, you can't live in those units, there are 2 out of 6 that are that musty (and only 2 of the 6 are currently occupied).

Any help is appreciated, thanks