The number of the CUs is not a problem (actually they are reported by OpenCL as 36). Something else is going on in your case. Make sure that all hardware control options of PhoenixMiner are turned off (i.e. remove all -cclock, -mclock, -tt, -cvddc, -mvdcc, etc. parameters), and set the clocks, voltages, and fans via the AMD control center software. You may also add the -hstats 2 parameter to see the actual clocks of the card when mining.