It sounds like your core bit is glazed over which means the diamonds are not exposed to grind the material. Run your fingers over the cutting surface. If it is smooth then its glazed over and all you do is run the bit on a concrete block dry to expose the diamonds. The next question are you drilling with or without water ? Diamond core bits need water to cool and remove the material. They do make dry core bits, they take longer to drill through the material and you need to let them cool down as your drilling. If you need to drill through hard fire brick then you need a core bit for that material. You have to use water with that type of bit. Who ever you are renting from they should know which core bit to give you for the job.
