I took bunch of pics throughout my first SC charging session. I noticed 48kw was the highest number it reached in 35 mins of charging. Also I was the only one there, other 7 stations were empty.. at Laurel, MD.
Your charge rate, starting at 60ish%, is not unreasonable. The battery will not be able to safely charge at high rate, the more full the battery is.
There are still some possibilities that can cause slow Supercharger rate, other than having a mechanical problem with the car.
Temperature can be an issue. The battery must be at an optimum temperature to accept the highest charge rate. If it is cold out, that will reduce charge rate some, particularly if you have not driven the car enough to warm the battery or have let it sit in the cold before starting to charge. I'm not a cold-weather owner, but others who have experienced this can be more helpful.
Constant Range Mode use can be your enemy. Related to the temperature problems above, Range Mode cuts back on the energy used to control the battery temperature. Possibly good for increasing range when you really need it, on a daily basis it is a mistake (that's why there is a switch to turn it on and off). Your battery will have to warm up while it charges at the Supercharger, and full power will not be applied to the battery until it is warm. By that time, it may have charged enough to be tapering off on the charge rate, so you may never reach full power. Also, battery longevity is somewhat dependent on maintaining battery temperature at optimum -- running constant Range Mode does not allow that to happen.
Most likely is that the Supercharger is having an issue. Try moving to a different unit (meaning a different NUMBER in the stall, not just a different letter with the same number). There may be times that the grid has less power available, again probably temperature related, where the entire Supercharger is not able to run at peak. Take what you can get and stay warm!