Several potential reasons, Google will cover most since they are all simply ideas that you need to check to confirm. Generally it's an issue with the master cylinder leaking past the seals, air in the system also as a result of first, leaking seal at a cylinder or caliper. Look for obvious fluids leaks, also look for fluid loss, or gain in reservoir where air has entered the system somewhere and stayed elsewhere. Did you bench bleed the mc before installing? Complete fluid renewal and bled from furthest point forward? Don't drive it until sorted.