I have put a 50 point bounty on my question If photons have no rest mass, where does Planck's constant come from? 3 days ago. 4 days are left before the bounty expires. However, I have not yet received an answer there I cannot understand why people are getting so late to answer my question during the bounty period.
This is just a thing that happens. A bounty increases the visibility of a question, but doesn’t guarantee that it’ll be seen by a person who knows a good answer and is willing to take the time to write it up. I’ve certainly had failed bounties of my own, both on my questions and on others’.
If your bounty expires, you can add another one afterwards. Be aware that subsequent bounties cost more reputation points.