I agree with the post about dehydration - make sure you're drinking plenty of water because I've found first hand that that makes a big difference - the only other thing I might think is the problem would be inadequate let-down. I've had a few incidents where I've tried to pump and gotten nothing because I never let-down. Either environment distractions (my 3-year-old screaming, etc.) or other stressors prevent me from having a good let down and I get nothing. Try to relax, think of baby, and be comfortable. I've also heard of warm compresses or showers prior to pumping to assist with the let-down. It might be the issue, who knows? Just a suggestion! Good luck!