Trimming Q's

This year my Shira failed to leaf on some of its bows. I call them bows but
in fact they are fairly small sprigs. Other bows extended/spouted and leaved
as normal.

I want to keep the tree fairly small and well proportioned, at the moment it
stands 1 metre high and has much of its leaf at the top, even more so with
the new sprouting. I would like its height to stay 'as is' but get it to
sprout/leaf further down.

How and when should it be trimmed.

dj