Also keep in mind it could be as simple as that they had a better candidate.
The issue is not just this one company. I have been at this for months with similar stories for a lot of them.
Sometimes I get the technical questions wrong, so that's fair enough, I understand why they'd decline me for that and as I said that's something that I can at least work on for myself by reading through textbooks and/or building sample projects to understand a concept a bit better.
A lot of the time, though, I won't get the technical questions wrong, and it's this recurring thing of "I really don't know why I'm constantly being declined".
Mock interviews are a pretty good idea though.