There are a few possibilities:
- The dye may have precipitated out of solution.
- Heat the GelRed® or GelGreen® solution to 45-50°C for two minutes and vortex to dissolve.
- Store dye at room temperature to avoid precipitation.
- If you are seeing high background staining of the gel, the agarose that you are using may be of low quality. Contaminants in the agarose may bind to the dye, resulting in increased background.