Yes, adding DME will increase ABV. Yes, you are missing things.
You need to consider yeast pitch rate. If you don't have enough healthy yeast culture to start with you will get a stuck fermentation. Also, higher ABV beers take time, sometimes lots of time. Both to ferment and to condition. If you are bottle carbing it is a whole different game if you have a high ABV beer that's been in the carboy for a while.
Also, why don't you try to make a high ABV style of beer rather than randomly adding fermentable sugars to a brew? There's plenty to choose from and it won't taste like ass if you do it right