The potency of the active substance may be limited by its solubility. Increasing the extract concentration won't increase the active ingredient concentration if the active ingredient has surpassed its limit of solubility.
Usually in disc diffusion method two things has to be taken care off.
1. The volume of the sample you are placing in the disc (pls. take care that disc is dried after adding the sample and has to be placed inverted in the plate.)
2. The solvent you are dissolving. Some times the concentration of the active components of your extract might be the reason.
Using different solvents (ethanol, chloroform, methanol, DMSO etc) may help in resolving the concentration question. It just could be insoluble or slightly soluble in the solvent (water?) i.e., saturation has been reached. But using different solvents and testing may help. Some time just increasing the temperature of the solvent during extraction may help. This would be double edged as high temperature may inactivate the compound. If you are using crude extracts of plants, there would be many compounds that may interfear with activity etc.