I am really confused about this. My question didn't get much attention even after offering the bounty.
What will happen to my bounty now? Will I be getting it back or is there any way to increase its expiration time?
What I can do?
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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.
Not much you can do I'm afraid.
The bounty is already taken out of your Rep when you first choose to place a bounty, you don't get that back.
You could place another bounty on it to get it more exposure, although It's unlikely you'll get anything better though because if it didn't get attention the first time around it's unlikely to get attention again, sadly.
Bounties are just a way to increase the exposure of that question, it's not a guarantee that you'll get a perfect answer. You've given the question some decent promotion and it didn't get you what you need, so unfortunately there's not much you can do. There is no guaranteed way to make your question get an answer you like.