[NTG-context] Odd column break - how to avoid?

Hello, In a two column document I'm preparing, I have a column break in a very odd place. The column just stops, and the document material just begins in the next column, leaving a lot of white space. I've no idea what's causing this, and I've tried to avoid it, but nothing I do seems to work. Is there any way of forcing a column not to break? Thanks, Alasdair