Less than three weeks is not a long time at all for a snake to go without food. Your snake isn't starving, relax

There are plenty of things to try.
First off, exactly how you are feeding the snake? If you don't already, I recommend you make the room very dark when you feed. One of my snakes refuses to eat if the lighting is too bright. If you can't get the room dark, cover the feeding tub (make sure snakey can breathe tho) and leave the snake alone with the mouse for a while (assuming you are feeding f/t.)
Again, if you don't already - try braining or slitting the frozen thawed mice (cutting the head or back, as the fresh smell can sometimes encourage the snake to eat.)
It's important that you minimise stress as much as possible between feedings. Avoid handling your snake as much as possible until he/she has fed.
Remember not to keep pestering your snake with food - once a week is plenty. The methods I have given you are by no means an exhaustive list, so don't panic if it still doesn't work out for you - plenty more tricks and tips for problems feeders
Good luck!