Eh someone else might have encountered this bug too so sorry if it's a repost...
Basically, I am simply unable to smelt my weapons at all, and am constantly failing with increasing frustration. I understand that I might be (a very very rare case if so) extremely unlucky. I have attempted to smelt my Dark Fledgling Bow and at least three other type of twin swords, and have even increased my success rate to 95% with no success. Is it...normal? The Beta smelting rates for lvl 10 - lvl 20 was quite good... What happened? ._.=

I believe you have just stepped into an ungodly amount of unluckiness with your smelting not seen for miles if that's the case. I smelt just fine with me succeeding where I should and failing where there is a roughly good chance of failing.

I have experienced a similar patch of bad luck after the recent maintenance/patch as well in regards to repair failure rates and smelting instances that have failed much more frequently that prior. Whether this issue is related I am not quite sure but it wouldn't hurt for some feedback about this from other players.

I think you are very unlucky bro, ^^v i just upgraded my weapon to +40 and repaired them with no trouble. my suggestion is to upgrade your partner's friendship. this might help you increase the smelting successful percentage and the repair percentage.

Thanks for ya replies! ^^
I suppose it could be my luck too...I have maxed Yoshi's friendship at lvl 5 already so I'll assume it's not the affection problem. =_= Eh, shall hope for the best.