I have a diverter control on my shower which controls a shower head, an overhead rain head and a handheld shower.

As long as either the overhead shower or the shower head are turned on, the handheld shower will have hot water.

If I turn only the handheld shower on, the water is either luke warm or cold. However, this happens most of the time but not all the time.

This is a Delta diverter.

My plumber keeps telling me there's trash in the diverter and I need to flush it out. I take the entire thing apart and flush it out about once every 2-3 months. Sometimes it helps slightly but usually not. This is not normal.

Does anyone have an idea why it's doing this? We have a whole house filter on our well.