Thermostatic shower bump start

[forreq]

Hi,
I have two thermostatic showers (different rooms) in a system arrangement.
Both showers have more than 1 metre of head pressure.
One shower has a pump and the other is gravity fed.
The problem is that both showers need to be turned to their lowest setting to start the flow.
Sometimes this is not enough and require a 'bump start' by allowing water flow from the basin hot tap.
Does anyone know if this is usual or whether something's away?

[plumbers1]

The length of the supply pipe is to long for the pressure to drop enough for the pump to start. You need a negative head pump.