Temperature Control Problems in Bedroom
I would check / change the distribution box next to the Air Handler and the duct size to that particular room. 

Engineering wise it was probably sized for the room not for the fact that it has 3 walls to the outside! 

Why over time has it become worse ? 

Good question now to the second issue you have.
Your Fan in the Air handler is not running as fast as it used to, bad bearings, bad capacitor or some other failure. Your system could have a blockage of sorts as well. 


I would change the duct size and also adjust the distribution box to move more air to that room. 
Check the Fan, the Fan bearings / check or replace the Fan capacitor.