I have a roof that has layer of shingles covered with tar paper. I have a leak which I cannot find? How do I find it and if I find it, how do I repair it?

In inspecting the tar paper over ths shingles, I cannot find any place that the water could leak.
I have some moss growing on the part of the roof. Can the water leak there?
Is there anything better to recover the tar paper than another layer of ter paper? And do I have to spread roof cement under the tar paper?