Sunbracers are very unreliable on actually proc'ing. This has been an issue for a long time and I had hoped that it would be fixed with Lightfall but sadly isn't. It being so unreliable makes running end-game content very tedious and frustrating. (Especially since Solar grenades can cure you now, not having the heal is very inconvenient)
I have messed around with my build and gear a lot to try and find the issue but can't narrow it down. It just seems to flip a coin and decide to either proc or not.
Note: Incinerator Snap is slightly more reliable to proc Sunbracers, I think this may be because it has more projectiles and therefore can hit more targets, increasing chance of proc'ing.
Current Build:
Aspects - Touch of Flame / Heat Rises (Though usually use Icarus Dash)
Fragments - Ember of Resolve, Benevolence, Char, Torches.
This seems to be an issue regardless of what you run. I recommend Dev team test running no Aspects/Fragments to confirm that's not the issue, I've heard people claim Solar 3.0 Scorch is the main thing that seems to have broken Sunbracers but unsure if that is true.
https://youtu.be/YGHQgGXzfJY | https://youtu.be/3yEU9l7pXMM
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Change to Incinerator Snap for more reliable activation.