Shower plumbing question

Hi all, i have a very old shower that has pipes visible, i fitted a new shower at the opposite end of the bath which to me makes more sense too as it backs onto the airing cupboard and i could hide the pipes.

I have attached a diagram of my set up, until the bath is swapped out in a couple of weeks i have left the old shower connected, to do this i've Tee'd off the existing run for the shower in the loft down to the new shower, eventually i've swap that to an elbow and cut the old shower off.

My issue is, the old shower is making the pump kick in, the new shower is not, it's a new salamander positive head pump, the showers are at the same height.

Am i missing something, i can't understand why only one shower on the same pipework triggers the pump. (obviously only try one at a time) The pump does kick in when i hold the new shower head in the bath.

An initial thought:
1. The pump turning on depends on the flow volume.
2. The flow volume depends on pressure at source and resistance en-route.
3. If the new pump offers higher resistance than the old, it may reduce the flow sufficiently to not trigger the pump.

Or the new shower could have narrower waterways or a shower hose/head that increase the resistance and reduces the flow to below the trigger level.

Need to check what flow the new shower gives without the pump running. does it activate if you drop the head to the lowest point?