Column Breaks and Keep options in CS6

I have a document with a text box divided into 2 columns. I have my basic paragraph style set with keep options to keep the first 2 and last 2 lines of a paragraph together. I have some cases, but not all, when the last 2 lines of a paragraph are pushed to the top of the next column, yet the column they are coming from has empty space for 2 or more lines of text. Why is this doing this? And how can I stop it? I'm useing CS6.