In my experience, it is really difficult digging around inside a chicken's foot, especially if you are afraid of damaging it, or hurting them, or doing something wrong. I think, like I was the first time, a lot of people are too tentative. Dig a little and give up, thinking its not in there, or I can't find it, or I'm going to mess this up and they'll never walk again!
Confidence and perseverance are needed. The first time I had to do bumblefoot surgery, I had to do it 3 times over a few weeks because I didn't get it out properly on the first or second attempts (thought I had but didn't dig far enough). Third time lucky! But it wasn't luck, it was me getting over my nervousness and being resolved to not put the hen through that again. I felt awful and inept, but finally cracked it and it was SO satisfying an achievement!