If you’re having this issue in Skyrim’s Dawnguard where shooting a bloodcursed arrow from Auriel’s bow has no effect, the reason is due to a small bug.
If you improve Auriel’s bow (as I did using Moonstone to ‘Legendary’), this confuses the game and treats Auriel’s bow as if it’s a different weapon, and the special sun-blotting power doesn’t activate. Particularly annoying if you want to unlock the achievement!
PC players can work around this bug by spawning in another copy of the original Auriel’s Bow with the console command: player.additem 02000800 1
Hopefully Bethesda will fix this bug in an upcoming patch….