Like my guru I teach very little technique. A huge catalog of techniques is nothing but organized despair. Simple tools widely applied, a firm foundation in principles and good attributes will generate all the techniques you could ever want and more.
Not all students will get everything. A good teacher will teach to the capacity and level of commitment of the student so that the student will have the chance to absorb all that he can. Push them past that point or give them too much too quickly and they'll end up with less. Of course, that's one of the classic ways of hiding stuff. Make them drink out of a firehose so they don't actually internalize anything.
There is stuff that you don't want to teach until you know the students really well and have a lot of trust in them. Guru Plinck will not teach certain aspects of attacking or any knife work at all to people who haven't been with him a long, long time. The potential cost is just too high. Some people can't be trusted.
In the end, if you pick and choose what to teach to whom based on some mystic sense of 'worthiness' or, as it more commonly falls out, how vigorously and skillfully one strokes the teacher's ego you do yourself, your students and the art a disservice. Plenty of systems have gone extinct because a few generations of teachers only transmitted 80% of what they knew. Good students get disgusted with the contortions they need to go through to get The Secret Knowledge (tm) and go somewhere with less BS.
Some of this comes from running commercial schools where you have your serious students and your rice-bowl students. You need to keep something just out of reach to keep them coming back. Part of it comes from fundamental beliefs about who is there for what. If you believe that the students are there for your benefit you will behave one way. If you believe you are there for the students' benefit you will behave in another.