The Eclipse needs a 10-25K pot with a TRS (stereo) connector and a TRS (stereo) lead.
Your symptoms suggest that the R of the Eclipse socket is not connected to the pot (this is probably what they mean by Mono) – check your cables and talk to the pedal manufacturer.
Using a 25K pot will help, but not fix the problem – you need a "stereo" pot and a "stereo" lead.
