if you use cgminer or bfgminer you need to enable CPU mining during the configure step, when compiling the code. check the configure script in the source directory to find the parameter to be set.
Also I noticed, that at least bfgminer doesn’t do CPU mining, if it finds a GPU,what makes sense, as you can expect a hash rate of around 1.2 -1.5 khash only for cpumining, while any GPU gives hash rates of 10-600 khash,so that the additional hashes comming from CPU mining are less than the variance you have in the hash rates comming from GPU mining.