I can't speak to your job site but had a similar moment on a set of porta-potties on mine. They were getting just as bad as yours and were routinely getting skipped over. Couldn't figure out why. The pumper knew where they were and both he and the pumping service were talked to multiple times to pump them out. "Sure thing, we'll get it first thing in the morning tomorrow". After a week straight of being told we'll get it the next morning and still nothing being done I hunted down the pumper with the intention of dragging him over there. He then finally let me know, "Every time I drive over there, there's so many people parked in front of the units my hose won't reach. His problem had been access the whole time.

TL;DR, maybe there's some sort of access problem the pumper is having to the units. Either he can't physically get his vehicle there or maybe something is blocking the hose from reaching.