Hamburger Menu not working on reduced browser size

Without any available code it’s a bit difficult to figure out what’s going on for sure; however, I’m guessing the problem is that there is some JavaScript code that activates the hamburger menu only if a screen is below a certain width.
The script only runs when the page first loads, so it is only then that the screen width is checked – in your mobile device the initial width is small enough, so the code runs, in your screen the initial width is greater than set by the code, so it doesn’t run.

The solution would be to simply locate the code snippet (in many themes it is inside a file called navigation.js) and remove the part that checks the width, so the code runs regardless of the screen size.