Hi Lexie!
For free entry text questions more often than not the trigger that tells the variable to populate from the text box is "when textbox loses focus" aka when the user clicks outside the text box.
The easiest way our team has found to work around the problem is to disable the submit button until the text variable =/= (blank) and then just have a little pop up or other instructions telling users to click outside the text box in order to enable the submit button.
You may also want to double check that your text box on the slide is filling the same variable that the slide is checking to advance.