Mobile Menu function disregarded when cached

Hi, I have a custom function in my theme to switch the menu from the standard menu to a mobile menu (changing which menu is loaded with a variable wp_is_mobile() statement). When the cache is clear, this function works, but after the cache builds up, the mobile version of the website ignores the mobile menu and loads the default full menu instead. I have put the file containing this function in the list of excludes, but it doesn’t seem to make a difference. Do you know how I would resolve this? Thanks!

Aaron