Frustrated, yes.
Especially if I've worked on it, given it a break, picked it up in a fresher mindset -- and still couldn't grasp it. Or if I've been breaking down the code and refactoring entirely and getting new, unfixable bugs. Or I decide to finally post my question on StackOverflow after finding it nowhere online, and getting people recommending solutions I tried 3 days ago.
But I live for that moment where I come up to a problem that's plagued me and solve it fairly instantly because of the different mental state I'm in. Makes me feel like there's no problem that's insurmountable without a good nap, yoga, or coffee break.