I think the thing to go for here first is to examine any filters installed in the pipework and fittings. Often if the pressure suddenly drops it is because debris has collected in the filters and stopped the flow. Switch the shower from full hot to full cold. Is there a difference in flow rates between the two? Does the pump fail to kick in on either? If so suspect filters.
Shower pumps are pretty simple devices, and if the motor is running you can be pretty sure it is pumping so look elsewhere for a fault first.
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