I have a Trek 1000 road bike and I have had 4 flat tires in my last 10 rides. It's really annoying because I have in the past gone 6 months or more without a single flat. In each case I replaced the tube and tried to scan for anything sharp. 2 of the flats were in the front and 2 in the back. I'm riding on the same roads I always have. What are some other things that could be causing a sudden increase in flats besides bad luck?