There are various possible solutions using Cogent™ HPLC columns:
1. Diluent: Is your diluent too strong for the mobile phase? You should not have a 90% acetonitrile diluent with a 10% acetonitrile Reversed Phase method. You may observe peak distortion for this reason.
2. Mobile phase / diluent pH: Do you have any basic analytes? If so, perhaps they may be responsible for the poor peak shape. You can operate at an acidic mobile phase pH where any residual silanols are neutral, avoiding electrostatic interactions with basic compounds.
3. Sample concentration / Injection volume: Every HPLC column has a maximum loading capacity. If too much analyte is introduced to the column, you may see peak distortion due to overload. Try reducing either concentration or injection volume.