My rear tire seems to repeatedly go flat. I can't figure it out. There's nothing in the tire, there is nothing poking through the rim. Am I missing something? Is there a trick to figuring this out?! I replaced it about a week ago; I replaced it again this morning. This afternoon, it is once again flat. I did two 8 mile rides (commute) today and that's all. Yes, there was glass and road crap involved. But enough to warrant another flat? I don't think so. And why is my front tire still perfectly inflated?