Luke wasn’t exactly a Master. Doing and teaching are also two different skills. Perhaps Luke wasn’t a good teacher.
I suppose that’s possible. If so, all hope is lost for the Jedi. Oh, wait – no. Rey is good at everything, so she’ll succeed where Luke failed. So maybe she was the “there is another”? Oh, wait. She wasn’t born yet. But then maybe Yoda could see that far into the future. We just needed Luke to carry us past Darth Vader and the Emperor and then let Rey take over from there. Or something.
As far as Rey’s journey to the Force goes, Luke learned almost as quickly. Granted he had a bit of teaching from Obi-Wan, but not much.
I’m not so sure I agree with you on this point. In ANH, Luke was only beginning to discover the Force. Then there were 3 years between ANH and TESB, and I’m sure Luke practised every day. In fact, he probably went to the MF, got his remote lightsaber training ball…oh, wait…no he didn’t – Finn found that still aboard the MF, right where Luke left it. I guess Luke just shadow boxed instead.