Open and close all windows and doors. When you secure the lock, is there any gap between the seals? A good way to check is to trap a $1 bill between the seals, lock it, and then try to pull the bill out. If it slips out easily, there’s enough of a gap to leak the heat out, and you may need to replace the lock or the seals.
To tell which part is the problem, firmly grab the handle and try to wiggle the door or window in its frame. If it moves, you may need to replace the locks, which should be tasked to a professional windows and doors fitting company.
Note: you can also use the $1 bill method on your fridge to check if the seal has loosened over time!