Why don't my composite deck screws go in all the way? Why RT screws?
I believe the intent of the reverse thread is to suck the deck board down onto the stringer.  You don’t want the deck board and the stringer both engaged to the threads; that will hold the deck board spaced away from the stringer and put all the weight of traffic on that thread.
If the screw isn’t going in all the way, turn the head clockwise some more.
How to do that exactly is a matter for discussion between you and your screwdriver(s).
It will either go down more, or break.  If it breaks, then the wood is too tough for it; consider pre-drilling the holes.
If it’s not descending, then the screw isn’t turning.  If the driver is simply hopping out of the miniature “socket” inside the screw head, stop that from happening immediately because it will quickly destroy the “socket” and make the screw un-turnable. This is most often an issue with Philips screws.
If your screwdriver is refusing to turn the screw any harder (without hopping), then you are at the torque limit for that tool.  Some power drivers have a torque setting at which they’re designed to quit, and that can be changed. Again, pre-drilling can reduce this torque somewhat.  Otherwise, change tools.
I prefer a “speed wrench”, which is a socket wrench shaped like a crank. I use it with a “bit holder” placed in the socket; this gives a good combination of power, control and speed. I’ve never found anything I couldn’t drive with them. Also, the speed wrench allows you to dedicate your drill to pre-drilling.