Spent 3 weekends trying to stop my patio from flooding and it was one dumb thing with the slope
Water kept pooling against my back door every time it rained, so I re-graded the whole patio with a rented plate compactor, added 2 inches of gravel, and still had a lake out there. Finally a neighbor poked his head over the fence and asked why my downspout was dumping right into the low corner, moved it 6 feet and it's been dry for a month. Anyone else solve a project with the one thing you should've checked first?
How did the neighbor figure it out before you did? I only ask because I've been there, where you're so deep in the problem you can't see the simple cause. You get set on the fix you decided on and you stop asking what changed. Was the downspout always dumping there, or did something shift it?