Bootstrap navbar-fixed-top class, not working

Because you are using the newest version of bootstrap (bootstrap4 beta6).

They change from .navbar-fixed-top to .fixed-top

You have to read document in this link.

I think there is something wrong with the way your are loading in your bootstrap. Make sure you call jquery before you call the bootstrap files. If nothing works you could always add the fixed positioning yourself:

.navbar-fixed-top {
    position:fixed !important;
}