How do I get a back to wall toilet with a concealed cistern to flush proper?

I have installed a back to the wall toilet pan with a concealed cistern. The base of the cistern is about 4cm above to top of the pan. Nowhere in the literature was there any limitation placed on height. The flush does not seem to have enough force and does not carry the waste away.It is a SIAMP 2118 floor standing back to waqll toilet and reend cistern.