My question didn't got much attention even after offering the bounty.
You've got four answers, and at least one of them shows a great deal of thought and effort. Perhaps it's not a perfect answer from your point of view, but it's a darn good answer and it looks like it was added during the bounty period.
A bounty is not a guarantee of the answer that you're looking for, it's just a way to sweeten the deal and attract some attention. It seems to have worked in this case just the way it's meant to. If you want even more attention for your question, you can always offer another (possibly larger) bounty later.
So, just award the bounty to the most deserving answer (the choice seems pretty obvious). Offer another bounty if you really think that someone might write an even better answer than the ones you've gotten already. Perhaps a better option, though, would be to think carefully about the points that you still don't understand, and then write one or more new questions that covers those points specifically.