If you are sure that the pump is running...
Check the inlet shower pump filters.
//If there is still poor flow after you’ve cleaned and descaled the shower head then it may be necessary to clean the filters inside the shower pump. The filters can become blocked with limescale in hard water areas and, after the initial installation of the pump or where a lot of work has been done on the pipework in a house, with jointing compounds, tapes, flux and other bits of debris. To clean the filters, turn off all service valves and disconnect the electrical connection to the shower pump. The filters are generally located behind the flexible connection pipes to the impellers. Remove them and clean them gently. Then reconnect the system and refill the pump by opening the valves.//
Or.
Is there an airlock in the shower pump?
//Should you suspect that there’s an airlock in the pump which is impeding water flow, you’ll need to bleed the pump. You can do this by turning off the electrical supply to the pump and then open the shower mixer and any other taps that are fed by the pump, allow water out and then close them again. You can vent the pump directly by removing the connection pipes after you’ve turned off all the valves. Bleeding the entire system should be a last resort.//