That "brain flip switch" thing is so true. Like @abby_brown said about old stuff, do you find this happens more when you're actively cleaning or just randomly? I once stared at a broken ladder for ages before realizing it could be a plant stand. Felt like a genius for five minutes.

Tried promoting from within once and the guy lasted three weeks before asking to go back to his old job, so my track record isn't great. An outside hire can bring new ideas, but you also risk getting someone who doesn't fit the shop's vibe at all. Maybe see if you have anyone internal who's almost ready and get them some quick, specific training.