I cannot answer your question as I rarely grow tuberous but here is a site that may be of assistance.

This is from the Scottish Begonia Society where they grow lots of tuberous begonias.

http://www.scottishbegoniasociety.co.uk/resources/Is+leaves+the+answer.No2.pdf

Thanks. Explains all. Tests start tomorrow.

as far as i know with tuberous and non-stops - they won't grow new branches from the node where the flower grew. so when you cut them for rooting it's important to include at least 1 bottom node that did not have a flower bud. Perhaps your did not have even one 'unused node' ?

what you might want to do is pinch the top 1/3 (i would cut the stem by half - it's too leggy) and that might force it to grow a few shoots from the bottom.

i can see a very small shoot at the very base - at the very least it will start growing faster.

make sure to pinch it so it won't be as leggy.