The challenges can certainly be tricky, especially when you are starting out.
One thing I like to do is run a couple of chapters ahead. So I don’t attempt the Challenges in, say, Chapter 8 until I have completed Chapter 10. That way, I find myself re-reading the Challenge Chapter and I come armed with a little more knowldege.
Another thing I like to do is find a supplemental source related to the Challenge. This can be the Apple documentation itself, one of the many online resources, the Stanford Online classes etc. It’s the nature of the beast that a beginners’ book can only cover a subset of material from a particular angle and sometimes that angle isn’t the one that clicks. So a supplemental insight can lead to the “aha” moment.
And don’t forget to make use of the forums on here. There is a lot of great information.