Have you been using psyllium? It may be that you're getting blocked up. It may also be that you don't have a lot to release right now.
I just did one, and I have to say that I didn't release much compared to other times, but, compared to other times, I actually had some regular BMs prior to the fast (I'm on day 3 and it's my first swf). My problem is probably the opposite of yours...I don't release enough on a non fasting day.
Do you eliminate frequently on ordinary non fasting days?