The colour production with GOPOD Reagent may be affected by high salt concentrations above 0.2M and possibly by ascorbate. To test for possible effects on colour development, analyse your sample with and without addition of known amount of glucose during incubation with GOPOD Reagent and determine whether the colour intensity is changed proportionally.
Also, if there are any acidic substances present in your sample, e.g. ascorbate, the pH of sample solution might not be optimal during incubation with the Lichenase. Some of the interfering substances might be removed by ethanol washing.