I agree with Alex, I've run into the same problem many times before.
Most often, it wasn't related to my purpose, other times, like what could've been the trouble with AndriDem, I had too much on my mind, the cause being a bad day in school, argument with a family member, etc.
If you feel your emotions are surpassing your intution, try not thinking. If there's one great thing I got out of the Reading Genius course, it's that thoughts are what cause stress, and are what help surpress our natural ability to learn.
Before activating or Photoreading, try clearing your mind of thoughts.
Breathe in deeply. Wait until you feel five heartbeats, or more if you can, then breathe out. Don't put your hand up to your chest, just feel your heart beating against the inside of your body.
In order to really feel the heartbeats, your mind is almost forced to quiet down. This, I found, is a great addition to the Photoreading method of getting into the ideal state of mind.