No, you can't increase the bounty once it's been set.
But don't assume that just because the answer rate has temporarily slowed that the flow has ended completely. Questions with a bounty are still shown on the "featured" tab, so the people who are likely to be enticed by the offer of a bounty will still be able to see it when they go looking for questions to answer.
If it's really a difficult issue, some people might be taking the time to try and figure it out before they post an answer. I know I often do this on some particularly tricky questions. I try to tackle the problem myself and then post a complete solution, rather than just a list of "ideas" or "things to try".
+50 seems like a perfectly reasonable bounty, and you only asked the question a few days ago. Getting a good answer sometimes takes time. We all get spoiled by how fast some questions get answered here, but that's hardly a rule we can assume to apply in every case.
One good thing to do is continually modify and update your question with some of the latest things that you've tried during the bounty period. That will have the effect of "bumping" your question to the top of the unanswered list, and it makes it appear more "active".
If, after the bounty does expire, you still haven't received a good answer, you can go ahead and set a second bounty for a higher amount.
Then again, you might call me skeptical, but I'd have to see some stats to convince me that a +100 point bounty really convinces more people to answer a question than a "mere" +50 point bounty.