from experience..... ive had a very similar problem with a previous tank. had an aggressive FOWLR
tank with a shark/eel and a few others. nitrates stayed at 80...... tried water changes consistently, after a couple of days the nitrates crept back up to 80, I even thought at one stage that all the nitrates being created were somehow doing it in the sump so I even completely cleaned out the sump but still no luck.
don't really know whats in your tank but could be too much stock for the filtration to handle or when the maintenance guys are doing water changes are they stirring up the sand as well??? that could be it.
I eventually just overkilled my filtration by doubling my sump and adding all these extra things to it. eg biopellet reactor, crushed coral in the sump, etc and now it just stays at 0-10 nitrate never changes.
this little bit of info probably wont help if your looking for a quick fix lol