This topic should be made a sticky.
Assuming there are no medical explanation (the pipes work well, you're not on some medication such as antidepressant that are known to reduce your libido and ability to cum, good mental and physical health, healthy diet, etc.), a very common explanation for your trouble would be your wanking habits.
Many guys tend to use a very tight grip when jerking off, and/or to not vary much their wanking routine. The side effect of this is that Jr. gets into the habit and becomes unable to cum from other, often more subtle or gentle, forms of stimulation. As someone else as said, a pussy or a mouth is not going to provide Jr. with the same firm and vigorous stimulation as your hand does.
So what do you do? Re-train your cock by changing and varying your wanking routine. Jerk off with a condom (assuming that you are going to use them at least for FS, it's a good idea to train your cock to enjoy the sensations through a condom), use lots of lube, loosen up the grip on Jr. use your non-dominant hand to jerk off, get yourself a
fleshlight, etc.
The key here is to be the bigger man, and to not give into Jr.'s protestations. It will most likely take a few wanking sessions for your cock to re-learn to enjoy and cum from different sensations and stimulations. But don't give up by reverting back to your old habits. After a few frustrating sessions, your cock will learn to take what it gets and to cum from it.