we keep getting called to a schools IT department where they complain of the units smelling. There are probably 8 or 9 split units in total, each in different classrooms.
The problem is with only a select few where the teacher keeps complaing " there is a smell coming from the AC"

The units are about 12 months old and like new. Each time we get on site, we spray coil cleaner on the indoor coils, rinse through the coils and the tray with water and then put a smelly jelly in the return air on top of the filter.

This works for a few weeks but then we get the call again saying they are smelling.
What causes some units to smell and not others. Is it stagnant water in the tray? Or is it something else?