The MXR Phase 100 uses Vactrol optocouplers as the variable resistance. This is different than their earlier Phase 90, which uses matched JFETs as the variable resistance.
Symptom: No phase effect
I've had this symptom in at least two Phase 100s. My most recent one was a dead Vactrol. Small Bear Electronics had a sub: the VTL5C3/2. I also remember a very dirty rotary switch killing the effect.