Zoom on MacOS Sierra

Go to System Preferences > Accessibility > General and be sure Zoom is check marked. Then go to System Preferences > Accessibility > Zoom and make sure your preferences are set to your liking.

If that doesn't help then boot into Safe Mode and see if it works as expected there:

Use safe mode to isolate issues with your Mac - Apple Support

If it works in the Safe Mode, try running this program when booted normally and then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor.  Please use copy and paste as screen shots can be hard to read. Click “Share Report” button in the toolbar, select “Copy to Clipboard” and then paste into a reply. This will show what is running on your computer. No personal information is shown. If the log won’t post, try posting it in Pastebin and provide a link in a reply. After pasting the report in a PasteBin page, go to the top of the page, and copy the address in the URL bar. Paste that in a new reply.        Pastebin

Etrecheck – System Information