This is an economical bike with a what appears to be a 36h Alex rim.

Simply replacing ALL the spokes with a reputable brand (DT, Sapim, Wheelsmith) 14ga straight and paying particular attention to attaining reasonably high and equal tension should probably fix the issue. If you or anyone else at the shop considers themselves a wheelbuilder but is unsure of building for clydes, aim for +/-5% with regards to tension variation.