If they're new, and working solo, then one should take that into consideration. In other words, if you see that they're trying very hard, but just don't quite have the knack of keeping everyone happy down yet, then I would tip normally. If you find them sitting on their keister in their room when your room is still waiting to be returned to daytime mode, then I'd probably not tip.
I once had an attendant on the LSL who was semi-new to Amtrak, but prior to this run had only worked the Superliners. It was her first run in a Viewliner sleeper, which does have differences from the Superliners. I had to show her a trick or two about things, like how much easier it is to hook/unhook the safety strap if the bed isn't fully down, for which she thanked me profusely. But otherwise she was working hard and doing her job, so I tipped her a bit more than the recommended norm, just because she was really trying hard and had otherwise met my expectations and needs.
If they're working with a seasoned attendant, then most likely I'd tip the seasoned attendant, unless by some chance that seasoned attendant was useless. If the newbie had somehow gone way out of their way for me, then I might slip them a couple of bucks separately, while still giving the lead the normal tip.