Marriott branded hotels refuse to extend stays for the same reason that you get transferred to Marriott reservations line when you call a local hotel asking to book a reservation, but then if there is a rate "glitch" such as a five night stay with the first two nights at $149, the third night at $299, and the last two nights at $149, but if they try to book it as three nights it comes up as $159 for all three nights, and then booking the other two nights for $142, which makes no sense! So, Marriott reservations puts you on hold to call the hotel to ask for permission to honor the $149 rate for all five nights, but needs to speak with the GM for that permission. Weird yes, but it really does happen that way. If I want to extend, I usually don't bother with the front desk at all, I just go online or call Marriott reservations and ask for them to add it to my confirmation number if they can, and if not, to create another confirmation.