I don't know if this was true pre-Patch 5, but I noticed that post Patch 5, Thousand Cuts seems to treat the lack of an offhand weapon as an IMMUNE attack. That's the only explanation I can come up with for why I see so many IMMUNE messages attacking an enemy that has no reason to be immune to a bow attack, including the rune buff. No such messages appear for regular attacks with the same weapon against the same enemy. I'll try and post a screenshot.
If I'm right, wouldn't that mean bow only gets half the attack value of DW, for same base damage?
It honestly doesn't really matter, as TC will kill anything you use it on, whether you are a bow wielder or a dagger wielder. There is no functional difference between the two using it. I've never seen the phenomenon you're talking about though.
Are you poisoning your weapon before hand? I'm pretty sure that will produce the immune description.
Seriously though, thousand cuts is so broken in general that fixing only one glitch won't fix it's use in general. For example thousand cut's takes no focus if the effect is blocked in part, or if it completely kills every enemy before the final animation goes off. (You will reappear at the starting location if TC is blocked) and can use TC again immediately.
Because what triggers the Focus cost seems to be the final hit, if it doesn't occur, you can use TC every fight and face roll the game.
I just tested both of these on Red Templars.