I've installed windows manufactured by several different companies including Simonton.
I'll assume you've checked with your builder, because I'll tell you right now that improper installation will make all windows leak.
Out of square rough openings that windows are to fit into, screwed or nailed too tight at the sides or on the flanges. Flanges not sealed, will pull the frames apart and result in leaks.
I'm not saying it's the problem in your case - but that would be my first bet.
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