I am confused. If your night time temperatures are 50-60 degrees F, why are you posting in a winter hiking forum? These are warm nights, even for the summer! I think the answers you are getting assume you are camping in near freezing to below zero F.

If you are cold between 50-60 degrees F, and are using any kind of sleeping bag at all, and even just a minimal sleeping pad, I suspect your cold has nothing to do with the sleeping bag. Only way I could see that would happen is if you had the top wide open, not cinched up at the neck.

50-60 degrees in near 100% humidity can feel quite cold. This is like camping on the coast north of San Francisco. I think it was John Muir who said he had never been as cold as in summer in San Francisco (fog season- steady 50-60 degree weather). If the humidity is causing your skin to get moist, that may be the problem. In these conditions nothing works better than good quality wool base layer (I like Smartwool).

Sleeping bags only insulate. If you jump into a cold seeping bag and you are cold and your body cannot warm the bag, you just stay cold. Be sure YOU are warm when you climb inside. I like to do a little walking around before I go into the bag. Then once warm, all bags lose some heat so your body has to keep generating heat at night. If you cannot generate any heat while sleeping, it could be a medical/metabolic issue.