Threadless stem keeps loosening I recently got a 2018 Felt VR30. I've been riding for a long time, but this is the first bike I've ever had with a carbon fork, and only the second I've had with a threadless headset.

The problem I'm having is that the stem keeps creeping up the steerer tube; this pulls the plug up as well. Every few days I need to tighten everything up, and tonight I noticed that the headset's upper cartridge bearing is loose in its race.

I am concerned about over-torquing the clamp bolts, but I'm doing it anyhow, because 5 nm just doesn't seem very tight. I've applied some carbon paste to the steerer, which seems to help a little, but it hasn't solved the problem. The stem (from Ritchey--not stock) has visible loctite on its bolts, but maybe they need more? It occurred to me that it might have a different stack height than the stock stem, and I've played around with headset spacers, but that doesn't seem to make a difference.
Possible causes:


Deformation of the steerer around the clamp area, possibly from overtorquing.
The steerer is cracking and hence lacks dimensional stability. I've seen this cause the exact issue you're talking about and it's scary, so I recommend looking very closely for it. Take off any spacers below the stem to get a better look when doing so.
Using a plug and topcap assembly that doesn't provide as much internal support as the fork is designed to need. Sometimes plug assemblies are treated as totally interchangeable and in practice this isn't really the case. If this is what's going on it can easily have already caused cracking. If it still looks good and the plug assembly isn't snug inside where the stem clamps, try getting one that is.
Plain old poor fit/tolerance between the stem and steerer. This is pretty unlikely. 
Using a single-bolt stem with the carbon steerer. This is a big no-no and could have caused the sort of deformation mentioned above.


Bear in mind that 5Nm, with or without carbon prep in most cases, really is plenty when things are working right (given common lightweight stems that call for 5, of course). It sounds like something else has gone wrong and I doubt that more torque is going to be the fix.