That happens to me sometimes. Check which sketch is causing the problem, e.g. copy the smallest size - stretch it - place it apart, then try to blend. If it is ok then you have identified the problem. Redraw the large one. with white arrow select all - right click then choose join. Should work. Sorry I'm 6 months late. guess you've figured out the problem by now.
Sometimes there is an odd point floating in the wrong place, this should show up when highlighted.
Happy Drafting!