One big factor has been the shuttle I'm using. For the new summer-weight fabric, I can't use my main shuttle because it is designed for thicker yarns. The shuttle that's designed for lighter yarns has a problem with the yarn jumping out from between the plates of the tensioner. I'll be weaving along, paying attention to a hundred other things and realize that the weft thread is loose.
My solution was to modify the shuttle so the thread will stay down in the tensioner.