Mine were gobbling down the shavings even 4 days after I first switched (I switched when they were about a week old). I went back to paper towels again for ten days or so, then tried again, same thing. Eating the smaller bits like they were treats. Covered up the shavings again for another 4 or 5 days. By then they were ripping up the paper towels with their scratching, so I had to do something different. I found that they were only truely going for it when shavings were tossed onto a different surface (like the back of the plate I used to prop up their feeder) or when they scratched through to the floor. So I put down an extra thick layer making it harder to uncover the bottom of the brooder and changed the plate for a flowerpot that‘s exactly the size of the round feeder. It worked like magic. Now they only eat their feed.
Good luck! And I second the grit. It’s probably not common in areas where chickens are generally free ranging all day every day. But if they are not, grit is essential, even for tiny chicks.