You should gently remind them via email or in person that you need the letter by a certain date. In general, you can thank them in advance for the letter that they are writing and this should remind them.
If they are unresponsive, start thinking about other people who could write you a letter. You can store up to ten letters on AMCAS and you do not need to use every one of them. Generally, you cannot have too many letters so get as many strong letters as you can.