I agree that Kung Fu belongs to no one, not you or your teacher, yet it can be possessed. 
But only by those willing to put in the time to acquire it.
There are plenty of reasons why a teacher will not give a technique until the time is right. For one thing, your tendons, flexibility and strength needs to be developed before some techniques can be done without causing injury.
This, I wholly agree with. Walk before running so to speak.
Teachers who hold back out of a lack of trust, well, that’s entirely up to them if that is what they want to do, then that is what they want to do.
Some people just simply don’t deserve to have the time invested in them because of their own inability to understand or their unwillingness to practice.
If a teacher has to show someone the same tech again and again and again because the student does not practice, then that student doesn’t deserve to move deeper and the teacher doesn’t need to suffer the idiocy of the student. This is a major problem in many schools and I have had the displeasure to witness this myself. Usually, these students will fall away like the chaff of wheat because they do not have the consternation to open themselves to the lessons and to truly practice with all their heart and focus.
Also, remember that teachers are only human and have their own fears and personality defects as each of us do. Some teachers just don’t like some students and withold information from them on that basis. Some students don’t like their teachers and eventually leave them to find a new teacher that they are more compatible with.
Some teachers do get outstripped by their students and some students will never go beyond their teachers. This is really not that important overall. The real importance lies in the perpetuation of the art.
How long does it take to know who you will be open with as a teacher? 1 year? 2 years? 5 years? 10 years? How long before you’re certain that you have a good student?
You can never be certain because people change. People change their whole life in the measure of a single day. Character changes in the space of one experience that could measure only a few minutes. That is life.
If you want an art to survive in fullness, then it is necessary to give it in wholeness and without reservation. If you take a student, then that should be enough to start them in their absorption of what you have to give. If you don’t want to teach a student, then reject them from the get go and send them somewhere else.
peace