Steve,
I think you might be seeing a couple of things that are pretty common with polypropylene and also with direct-glue installations.
Poly carpet cleans up very easily, so it is fairly common to move too fast, getting just the surface soil. Carpet looks good, move on. The problem is that there is still a lot of grunge down in the carpet to wick back up, so you either have to clean more slowly and thoroughly or use very little moisture to guarantee the carpet will dry fast. Of course, if you surface skim spots will show up again when a tenant tracks in water.
Direct glue installation doesn't allow for any air flow through the carpet backing, so a lot of the deeper soil is more difficult to remove. The above still applies no matter what the face fiber might be.
And last, one of the peculiarities of poly loop commercial carpet is the way it compresses. People use the term 'lays down and stays down' but loop construction just smashes down. If you look at a new piece of loop carpet from the side, you'll see a nice upside-down raindrop shaped tuft, with spaces between rows. Once in use, those raindrops turn into flat-topped upside down triangles that inter-tangle with the next row. This creates a semi-sealed channel between rows that doesn't allow for much evaporation, so the day following cleaning traffic will squish water up from carpet that felt dry and cause resoiling. Use of air movers are required in these cases.
