That's a tough one, at least for me - I can't reproduce the problem on my instruments.

You do it like indicated on Multimedia file viewing and clickable links are available for registered members only!!  You need to Login or Register!!
fingering chart, right?

The only reason I can think of is that for beginners, half-holing is generally very difficult to handle right - you're on track if you can hit pitch.

Half-holing is a common, yet imperfect way of producing half-tones, so there is an alternative: You could try to "lip" it down. That should be doable in the upper octave - just play an A and let it drop. But it's much harder to hit pitch that way...

M.