Question about using black wet patch on leaking roof?

I live in an old house with a rubber membrane for a roof. Earlier this year it started leaking. A very slow leak, but a leak can cause big problems. I bought some black wet patch and went on the roof to see what was happening. Right over the spot of the leak was a finishing nail sticking out of the roof. Since I have sunk so much money into this house and have a lot more to spend, I did not want to call a roofer, so I put the nail back in and plopped some of the black stuff over the nail and waited until the next rain storm.

It worked for about a month. After which time, it started to leak again, but only for about 30 seconds. The drops of water were bigger, but it stopped pretty quickly. After a couple of weeks, the leak was back to how it was before. I went back on the roof to look at it again. I could not see any potential leaks in my patch work. Nor could I see where else it may have been leaking. Since the leak is in the exact same spot every time it rains, I think it is safe to assume that the water is not running along a stud (it is very difficult to access the attic).

For any of you who have experience with this, what could be happening? Since the first application did stop the leak, it would seem to me I did find where the water was seeping. Is it possible that water can penetrate black wet patch? Does wet patch not work well on rubber? If so, is there a better product I can use?

Again, I have put a lot of money into this house and I have a lot more to spend. If I do call in somebody, I want to make sure that it is the only thing I can do to get this fixed.