You are using different versions of bootstrap (v2 and v3). They probably override or interfere with each other. You should use only the latest version (not the combined one). From what I know, bootstrap v3 has responsive design included.
From bootstrap:
Bootstrap 3 is not backwards compatible with v2.x.
Taken from joshunts answer:
The file bootstrap-combined.min.css is for Bootstrap v2, it is the
combined css file of bootstrap.css (the main css file) and
bootstrap-responsive.css (the responsive Bootstrap styles). It's
exactly the same as using those two files separately but it just saves
you an extra request.
Conclusion
bootstrap.min.css = compressed version of bootstrap.css
bootstrap-combined.min.css = bootstrap.min.css +
bootstrap-responsive.min.css