How go get Dreamweaver split screen to show the page correctly according to its bootstrap layout?

Just an idea, without seeing the page itself...

Another possibility: It may be functioning as the designers of the framework intended...

When using Split View in DW, if you have the Code and Live set to Split Vertically under View > Split (instead of Split Horizontally), the window may be too narrow to show your full screen column behavior as it does in a wider browser window. Essentially showing you something more like what you'd expect to see on a mobile device within DW.

If you have your Split set to Vertically, there may simply not be enough room available.

Alternatively it is always good to test the site online or with a testing server outside of DW. If you can share a link to a page in question with the issues we could help examine the page live for you.

Save document.

Toggle from DESIGN to LIVE with Ctrl + Shift + F11  (Cmd + Opt + F11 on a Mac).

If that doesn't work, validate your code and fix reported errors.

Window > Results > Validation. Check document.