How to fix a leak under a window?
First of all don't recaulk.
Find the leak. It could be above the window.
I suggest that you wait till it stops raining. Then take a hose and run it on slow. Go SLOWLY up one side of the window till it leaks. If it doesn't leak then go up the other side. If none of that leaks then go over the top.
Once you find the leak then deal with that.
I fixed a leak for a guy who had replaced the window after recaulking many times. He finally called me and I found that his leak was at the gutter where is was blocked and overflowing behind the siding.
My idea of a perfect day: No where to go and all day to get there.