What happens when you put her in the cot? Is she happy in there? Does she just play for the time she's in there? If so, I would just leave her in there, and go and have a nap yourself! At least she's having a rest, and you know she's safe. My dd would go through times of not actually sleeping at nap time. Now she is in a bed (at 20m) she does still play sometimes, but I find I can go in to her room, and remind her that it's not play time, it's nap time, and put her back in bed, and she eventually does go to sleep.