Window Leak Troubleshoot

The window is in a 1/2 story extension. it's a double window with single hung. it's a new window. when it rains it leaks on the inner frame - more spec. the drywall gets wet on the lower part under the verticle casing.
On a dry day, after removing the casing and drying out the drywall i set my hose to simulate rainfall first on the window itself for an hour - no water. i wait an hour then i do the same but include the top of the window where the window meets the siding (both vinyl). leave it for an hour - no leak. i wait an hour and repeat the same in the roof area over the window - run for an hour - no leak. i assumed i saturated the areas well and assumed i covered all possible entries. no leak. next day we have a moderate rainfall with no wind - after 1/2 hour i get the leak? anyone have an idea?
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Maybe the leak is above the window, a roof or side leak
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spray up at the eaves were the roof meets the overhang. If that causes a leak try caulking .

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