It’s only ever happened to me with one particular partner and that’s because he suffered from hypertension/high blood pressure and his meds for it made it even worse. What was tragic is every time it would happen (not happen) then he’d be upset which made him more anxious for the next time as if he was waiting for it to fail and so it would.
If you are young and healthy and playing with cornbird jr. on your own too much then he might become a bit desensitized to partner sex or get confused at the difference in pressure and buildup delivered by your hands rather than another person’s body which moves all of it’s own accord.
To rule some things out then just up your health watch and cut out as much salt, sugar and alcohol as you can stand. When you play with conrbird jr. then try lightening your touch more and more, slowing down to see if you can teach him to get off that way. In your mind try to picture the partnered sex act, take your mind to recall what it feels like inside a body rather than your hand.