There are a few possibilities that are dependent on your system, which you may want to examine. 1) Check the flow rate of the process (Trayed mercaptan extractor may be foaming)
2) Double-check the lean caustic concentration (manually)
3) Check the water concentration and quality in the water-wash
4) Ensure that sufficient time is allowed for water/LPG separation
There are probably more areas that you can check.