You "may" be experiencing brake fade. This happens when your pads heat up the pistons and the fluid boils. This can be made worse with older fluid as it's hydrophilic (attracts water from the air). The water expands as the fluid heats creating dead feel at the pedal/lever, pumping a few times may restore feel/pressure but it can be spotty--very similar to what you described.

The ways for this heat-up to occur vary--your brake is rubbing the rotor constantly (perhaps the return is blocked by debris), you're slightly riding the brake out of habit without realizing it, etc. etc.

The fix is pretty simple. Flush and replace your brake fluid. You might want to opt for Race-Spec fluid, which has higher heat tolerances. While doing this, inspect your pads, rotor and calipers. Check that there's nothing jammed between the pad and the inside of the caliper that could be preventing the return. Replace any other parts (pads, etc) as needed.