antenna is determined more by radiation pattern (gain and efficiency) rather than S11 as normally we consider.
My last resort will be, check the impedance again. If you able to recall EM theory (on plane wave theory), impedance and EM field are co-related. There should be a miss-match in your impedance between cpw to mstrip line, or mstrip line to antenna (not a lot) and this cost small reflection which effect the gain and eff and not S11 , tune or optimize your CPW or Mstrip line to get your desired goal. On the other hand you never change anything, you just add Mstrip line, where else could go wrong.