How to fix a leaking roof ?

We got an extension completed not too long ago, and a series of storms has been comming eversince late November. It has rained heavy maybe 10 times since then and including tonight. and 10 times we called the contractor to have it repaired, but 10 times he failed at repairing the source where the water enters. He has been adding more shingles, but it seems useless because the leak gets worse. Tired of calling this stupid idiot who doesnt know what he's doing, so what are things I can do to repair the leak in the roof ? btw, my prediction is that water is entering at the point where the extension meets the main house. what are some things I can do to secure that point ?