The biggest thing I see when mentoring new iOS developers is that they are struggling with basic Swift. The person might actually be able to construct a UI in a Storyboard, even with complex auto-layout, and get it working, but then not be able to add anything beyond the most basic features.
This is because a typical starting point for many new app developers is to find a tutorial that will take you step-by-step to building an app. These tutorials will actually do that, but when you have finished it, you haven't actually written any Swift. There's Swift code, of course. But, you didn't write it—you read it and copied it. And reading Swift, while important, isn't enough.
Like many things, there is a progression from knowing absolutely nothing to expert level. I don't think you need to be anywhere near an expert, but before jumping in to an app tutorial, start with Apple's official Swift book. I wrote a post recently on what I thought were the essentials.
The important thing is that you don't just read the book. Open up an Xcode playground and actually write code while you are reading.
Over my next few posts, I'm going to go through the Apple Swift book chapter by chapter and offer some exercises to try in an Xcode Playground. By the time you are done with the series, you should know enough Swift to try out tutorials and then actually progress from there to your own app.
If you have any areas in Swift programming that you struggle with, let me know.