It sounds to me like you've attracted a few players, but you can't seem to get them to actually comment on the thread or to do anything else other than actually vote in the poll. Is that correct?
If so, then what I would emphasize is simply encouraging people to comment. I always post up my interest checks before the RP is fully developed, so that there are still a few holes in the lore, plot, or setup that I can then use as discussion-starters. Posing a question, and asking potentially-interested players how they feel about a certain idea, is a good way to help players feel more engaged and invested in the thread. And getting an actual discussion going is great for helping players feel more invested.
In fact, now that I think about it, part of the problem might be that you chose to do a poll, rather than having people verbally answer in the thread itself. A poll is far less involved -- and, consequently, less engaging -- than a verbal response. It's easy for anyone who visits your thread to simply choose whatever answer best fits their vague feelings on the subject, without needing to think any further on it.
If your RP is already fully-developed and you don't have any good talking points, then, I would at least put a line in the interest check that says something like "And don't hesitate to speak up if you have any questions or concerns!" Generally, it's good to emphasize that you want players to ask questions and all that. Questions are good! They help players get more invested in the idea, thus making them less likely to only glance at the thread, say that they might join, and then not think about the RP again after that. And they help you flesh out your RP's lore, on top of that!