It is important to pay attention to your volume during playing for many reasons:it affects the efficiency with which breath becomes sound and your range of expression, for starters.
It is also true that your tone will be good when you are playing with high volume(because you will be blowing efficiently).
However,there is also the phenomenon of paying so much attention to your sound volume that you become unable to produce much volume at all. There are days when you aren't playing very well when the volume won't rise no matter how much energy you put into the flute.What is happening is that you are ignoring tone too much at the expense of volume.
Try blowing Ro for one solid breath. You will find that not only the volume but the tone varies continually. Over the course of this one breath anybody, no matter how bad of a shakuhachi day you are having, will have good tone for at least an ephemeral moment.
Don't let this moment pass unnoticed and unexamined. Instead, try maintaining this good tone and raising your volume at the same time.
This moment of good tone might very well come at the very end. This is because tone color will often improve when we are not using excess force.
When this happens, capture the shape of your lips and the inside of your mouth during this moment, then try to become able to replicate that at will. As you try this more and more, you will find your bad days becoming fewer and farther between.