difficult to advise without actually seeing it but perhaps the best option would simply be to remove all unsound material, apply angle bead and re-render.
is it only the render that's breaking up and coming loose? if it's the lintel then it's a much bigger issue.
while you're at it i'd be inclined to chip off a fair bit of the render above the windows by a course or two and have a look at what's going on beneath. may be some movement causing the render to come away.
Tapping the render around the problem area with your knuckles is a good way of identifying whether the render is spoiled and lifted, a hollow sound will confirm this
All affected areas will have to be removed and repaired.
As Chris said, if it turns out to be a problem with the lintel its a bigger issue.
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