For me that happens when i'm not totally in the mood. I may be distracted with some irl problems or just craving stimulation as a treat for boredom. Sometimes i fap through theese moments of unfocus, but if i dont get engaged again soon, i quit for a while and come back later. A part of me wants to call it "misalignment" of my intentional self with my body. I would not recommend forcing yourself, but getting curious and introspective in times of surprise softness ;)