1) Your (second set of?) questions turned out to be more time-consuming than the responders thought they would be, based on your original contact. So, they don't want to do it, but are too cowardly to tell you, and they are hoping you will just go away. If you can determine that this is the problem, then maybe you can make your questions simpler or fewer. In this case, your follow-up email would have the perfect excuse: you are sending them an easier questionnaire.
2) Your (second set of?) questions turned out to be more specific than the readers thought they would be. Unless they know you personally, they may even be afraid that you are a spy. (Do a search on "humint" and you will see why.) Even if they know you're not a spy, they may have asked for permission at work and been told, "No way! This is proprietary info." In this case, make your questions more generic, if you can. And again your follow-up email has the perfect excuse.
3) They are just typical lazy/busy people. You are low on their priority list, so you keep getting put on the bottom of the stack. In this case, you'll have to be a nag. No getting around it. Just be a nag in the nicest way possible, of course. Pity might be your ally. "Out of 20 people, nobody has responded yet." Even better, combine that with some peer pressure: "Out of 25 people, only 2 have responded." Then you can keep sending reminder letters with the increasing count of respondees, to ratchet up the pity/pressure. (You won't get 100%; don't even try.)