How can I find the source of a leak?
The best way to water test a roof is to have one person on the roof with a hose and another inside watching for water to begin entering.  The idea behind a water test is a process of elimination.  You want to start the water test at the lowest point of the roof closest to the interior leak.  Turn the water on, wait 15 minutes, then start moving uphill and in a progressively wider arc every 15 minutes.

Considering that you've already coated the roof with elastomeric paint, it's likely that the leak could be coming from the edge detail (coping metal, drip edge, fascia, etc), a flashing, or even through the frame of the bay window itself.