We have used a Chariot Couger bike trailer for a few years now. I rarely had to fix flats before. However, now one of the wheels seems to get a flat after every few times it is used. The flats never seem to be noticable when riding, but often show up at the start of the next ride. Every time I've looked it seems to be a thin cut on the valve, right around the area where it meats the frame. Is there a good way to prevent these flats. (This uses 20 inch schrader tubes)

Valvestem hole may have a metal burr. If it does, file it off.