At different times, I replaced the whole switch assembly or just the cam in my 65 Newport and 69 Dart. I recall that the plastic cam does help hold the position by clicking into place. A rod on the steering wheel bumps an arm, which unclicks it. So, yes your cam alone could be the problem, though usually it is just the ears that break off. Until you pull the wheel you won't know. At least buy a cam, because you will need it eventually anyway. It is much cheaper and easier to replace just the cam. I recall the early ones (my 65) are PN "SM9" and later ones (my 69) have 2 wires on them and are maybe "SM12". You used to buy them in Help packs at auto parts. Once I put a new switch for my 69 in my 65. I just didn't use the extra wires. I also recall using a later cam on an early switch. You can buy the cams on ebay <$15.
The whole switch assembly costs ~$100 new today. I recently got one cheap at PickNPull, from a ~89 Dodge motorhome. Those tend to keep the same switches and connectors much longer, and many pickers overlook them. When you swap one, don't cut the connectors off like most fools do. The pins are easy to release from the housings. And save the housings since you will never find them new. One guy currently asking on FABO thinks he can buy one.