I was also facing the same issue and the only thing that worked for me is the change in hierarchy of my project's folders. For better understanding, below mentioned are both of my project's folders hierarchies:
**Earlier(problematic):**
-Public
--CSS
---Bootstrap
----bootstrap.min.css
----bootstrap.css
--Fonts
--Images
--JS
**Latest (Solution)**
-Public
--CSS
---Bootstrap
----bootstrap.min.css
----bootstrap.css
---Fonts
--Images
--JS
In short must place the 'Fonts' folder besides 'Bootstrap' folder!