Hi Tim
The brakes are a simple hydraulic circuit. Pressure applied at the pedal is transmited to the rear of the brake pads pushing them onto the disc.
So there are a limited number of reasons for your symptoms. Firstly the brake pedal is not compressing the fluid inside the master cylinder. If that part is working OK and there are no leaks in the system the hydraulic pressure must be passed to the slave cylinder. Unless that is seized the pressure will force the pads against the disc.
If all parts are free and connected with no leaks, applying pressure at one end will make the other end move.... unless there is air in the system somewhere. Air is compressible, brake fluid is not, so if you have any air in the system it will absorb the applied pressure rather than transmitting it.
No leaks suggests an air-lock somewhere.
Regards
John