A bit more information on the separation conditions you're currently using (and the resolution obtained and perhaps the desired resolution) might be useful to help you out!
If you have tailing factors lower than 0.8 (peak fronting), possibly the column is overloaded, try to decrease the injection volume or sample concentration. On the other hand if you have tailing factors greater than 2.0 (peak tailing) there are secondary separation meachanism taking place during elution. If that is so, try to change column and guard column, if no improvements have been observed try to use a volatile basic additive to pair with silanol groups from the column and prevent secondary interactions. In addition, if you are only interested in increasing resolution by increasing theoretical plates try to use as injection solvent (the solvent in which your analytes are dissolved) your mobile phase or change column temperature as previously recommended.