You are probably doing nothing wrong except perhaps having too high an expectation.  For all the bells, whistles, computer printouts and controllers, irrigation is still more art than science.  if you are concerned you need to set our multiple identical gauges, or cans, or something along the length of the system.  A partially plugged nozzle, or w wear pattern, or any os several things could be causing a variation that make you think the system as a whole is wrong.

Having said that, if you are setting a system for 6/10 and getiing 1.5/10, something is definately wrong.  I would suggest talking to the local dealer to start with.  We can offer ideas and suggestions, but trained experienced eyes beat us every time.

We have one system, a Valley towable, that will walk faster one way than the other.  Nobody has ever been able to explain why with any reasonable certainty.