A Question for Brass Experts

I had a thought today when I was teaching my beginning trumpet students.  They have been stuggling with buzzing and producing a good tone.  I believe that they have a handle on how to buzz, since I have them do that for me without the mouthpiece.  Here’s where my question comes in…I know some brass players have to angle their mouthpieces down in order to get the most effective buzz.  How do I know when to direct a student to try that technique?

On a somewhat unrelated note….Here’s something that I’ve been thinking about for some time.  When do you decide to try to switch a student to a different instrument so that they may be more successful?  Do you have students that you find are struggling on one instrument, and then switch them to something else? When do you determine that you’re going to do that?  In other words, how much time do you give the student to be successful on their instrument before moving them somewhere is that you think they may have more luck with?