On your next long run weigh yourself before and after and note the difference. this might be enlightening also. I've read that you need to not lose more than 3% of your body weight…..that really isn't much. And during a race, are you someone that doesn't slow down for water? sometime actually try and estimate how much of each cup you actually get in your mouth. Say it's an 8 ounce cup, it's probably filled to the 6 oz mark and then you spill another 2 ounces. If you're average, you need to get 2 of these every 15 minutes, if you're a heavy sweater, maybe more and might even have to consider electrolytes (salt).
more miles per week and not getting most of the on course fluids up my nose helped me a lot… I had to practice drinking from those little cups… and I was also amazed at the weight I lose when I weighed myself before & after a long run