I'm assuming this is for a tub/shower?
There is a limit adjustment on most shower valves to prevent you from making it too hot and scalding your grandmother or little kid - big people should be able to be smart enough to check before getting under the shower!
Anyways, it's different with each valve, but normally, you need to take the eschution off and it will be a screw adjustment, a mechanical stop or something.
If you check the manufacturer's webpage for the model you have, you should find that on their installation instructions.
If that fails to resolve it, the spool valve could be stuck, or you could have debris in the inlet to the valve. Crud on the inlet is fairly common, especially on new or remodeling. It doesn't take much to restrict the water.
Another thing could be if there are shutoff valves to this, you may not have opened it entirely. Or, if you have galvanized steel pipes, the insides of the pipe could be occluded with rust, and you'll get very little flow.