Master page code behind doesn't fire from home page but fine elsewhere

I'm stumped as to what to show you in order to solve this.
I have a button on my navbar that loads a bootstrap modal search form. This much is okay from anywhere. The user makes a selection and then clicks the search button which is supposed to create the search and redirect to the search results page. But if the user does this from the home page the code behind does not fire - I've checked this with breakpoints on the first line. If the user performs a search from any other page it works perfectly okay.
The navbar and the search form modal are all on the master page. So the code is the same for any page this is done from.
I'm probably missing something entirely obvious but I can't think what it is.