Mujitsu and Tairaku's Shakuhachi BBQ

I've recently gotten to a point where I can somewhat reliably get my flute to play along. Now that I can play with different techniques and get some results, I've noticed that there are some unique problems that only appear with certain notes. With all flutes being different, I figured that these things could vary from one instrument to another. Until I can finally get around to buy another flute (longer, this time around), I only have one instrument to try and thus I can't tell if something happens with all flutes or just this one. Thus, I'd like to hear from others as to whether you also experience these same things or not.

First, when playing low octave ri, I can only get a sound in perfect position. Holding head even slightly down or sitting in a slightly bad position causes the sound to disappear instantly or turns it into a whistle-like sound that is highly chaotic. I found this strange, since doing the same with any other basic note results in only small quality or volume loss. Ro, for example, can be played very badly wrong without much loss in quality. The amusing side-effect of this is that I can always test that I'm holding my head properly by blowing a ri. This issue comes up occasionally if I get tired during playing and let my head hang slightly.

The second oddity that I've noticed over time is that high octave ro is incredibly hard to do by closing all holes, whereas any other high octave note can be done fairly easily using the same method. If I start blowing all high octave notes from ri and go all the way down to ro, everything else sounds reasonable whereas ro either gives me nothing or occasionally a somewhat distorted sound that is very unsteady. It is very difficult to make a ro that way, despite all my best efforts to learn the trick to it, so I try to avoid the ro with holes closed if possible.

Are these two issues common with all shakuhachi, or is it just something that is specific to mine? Or am I simply doing something wrong? I used to account these problems as flaws in my technique, but I find it strange that my technique would work just fine for everything else except few cases. I can already compensate for those things, but I'm curious as to what the cause is.