First, did you make the reservation within a day of the stay? Marriott does limit the guarantees if you make last minute reservations.
Second, is your MR# in the reservation? The guarantees require the hotel to know that the reservation is for an elite member.
Third, are you still checked-in to the hotel? While it is not impossible to get a hotel to honor a guarantee, it can be a lot harder and in some cases the guarantee expires if you haven't invoked it before you check out.

So if you meet all the above, I'd suggest contacting MR directly. (Call Customer Service.) Let them know exactly what happened, why you think the Guarantee is applicable and that the hotel isn't honoring the Guarantee. Most likely the hotel gave away your suite room thinking you were a no-show.

Be a bit careful in how you present the Guarantee. The hotel will want to focus on the "room type" guarantee (i.e. you reserved a king bed and got a king bed.) Focus instead on the fact that you had a guaranteed reservation for a suite room and they either oversold it or gave it away to someone else.