My dog generally will not use a new dog bed for several weeks/months. I don't know if it has to marinate, earn his trust, or what, but it usually takes a while before he will use a new bed. Sometimes the beds never pass his unknown test. He got in this bed the day we received it and has used it regularly ever since (although only in the day time - apparently it doesn't pass his night time criteria LOL). He is a senior, 40 lbs and I got the 36" and it is a good size for him but I would definitely recommend the next size up for a dog bigger than 40 lbs. As other reviewers have said, my only wish is that it had more support or came in an orthopedic version for the old pups.