This post from Mike Mason may explain the increased difficulty in getting online survey responses ...
1. Too many of them, from too many organizations;
2. Don't know how many questions up front, so end up frustrated, and stop. And then assume the next survey has pages of questions too.
Due to technology, I believe there's a significant third reason:
3. Many are reading email (i.e., survey email) on a cell phone/blackberry, and never go back to the survey on a computer.
Two things I'm going to try:
1. Always give the number of questions in the survey up front;
2. Sign-up for the new Zoomerang mobile option so possible to respond by mobile. [Note: Zoomerang is the online survey software I use - there are others.]
Hmm .... What else would work?