What was your lagging muscle and what exercise finally made it pop?

My biceps were always behind schedule even though I worked them just as much as every other body part. It wasn’t adding a new exercise as much as moving biceps up to the first exercises of each workout that I started noticing a difference. Prior to that, biceps were worked towards the end of my upper days and I guess I was too gassed to put in enough effort by then.

Now it’s my abs. I’ve been skinny all my life and still yet to have any type of “pack”. I wish I could just cut weight and get them to show up. I guess I’ll keep progressively overloading and hope they arrive one year.