New Pumped Exposed Aqualisa Quartz Digital Shower not very powerful?

We bought and had installed a new Quartz Digital Exposed Pumped Shower in about July 07, and since then apart from a few weeks, the water has not come through it at a very powerful rate, even with the extra burst button pressed doesn't improve too much. In the few weeks when things were rather better, they were brilliant and there doesn't seem to be a logical explanation about why things were rather better for short period. We've tolerated it until now, but want to know if there some adjustment that can be made to the control box. I cannot seem to locate the booklet which came with it, which might help. Has anyone got any ideas?