I, too, am sole developer on two huge projects. What you describe happens to me sometimes, too. I'm assuming that you're referring to those "stalls" that can last for days, and not simply battle fatigue.
I have found that the following things help me:
1. Pick out something very small and very simple to accomplish, and do it. Don't worry if it's not on the mainstream of the effort. What's important is to move forward. That relieves some of that horrible inner pressure that builds up when stuck for too long.
2. If that fails, try creating on or more small experiments for yourself, regarding things that you're curious about or things that have been particular troublespots. These can bring great joy and release the flow for you.
3. If these things fail, be productive doing something else altogether. Don't worry about wasting time, because you weren't moving forward anyway. This, too, can start a flow.
Remember, friend, that in a time of famine, a small harvest is better than none. Do not despise small accomplishments.
We should have exhausted this question when cigarette companies were caught enhancing nicotine levels in their product, in order to increase addiction, which enhances profit.
Consumers must have the opportunity to make conscious choices and give informed consent when consuming anything known (or reasonably suspected) to be habit-forming. I have no desire to form a nanny state, but folks need to know what they're getting.
You can tune a piano, but you can't tuna fish. You can tune a filesystem, but you can't tuna fish. -- from the tunefs(8) man page