There are a few reasons your beer could be under-carbonated.
1. Make sure you are carbonating at room temperature. You want to make sure that once you have bottled your beer, they stay out of the fridge while conditioning. Cold temperatures cause the yeast to go dormant - causing your finished bottles not to carbonate.
2. Your beer could just need a little more time. It can sometimes take a bit longer than two weeks for you beer to carbonate.
3. Your beer may not have fermented fully. If, before you bottled, you noticed a fair amount of sediment (gunk) around the top of your jug (above the beer line), then your beer likely fermented properly and your yeast should have been active enough to carbonate your beers. If you never saw any activity, your yeast may not have had the right conditions to ferment and carbonate your beer.
4. Sugar. the amount of sugar varies by recipe. You should confirm that you added the right amount of sugar. That can be honey or maple syrup.
5. Bottle Seal. There are two great options. The first is to use regular 12oz crown cap bottles (like these) along with a bottle capper. The second is to use swing-top bottles with a rubber gasket. The key is to make sure whatever you're using to bottle is rated to hold carbonated beverages. Jars, some decorative bottles, and growlers shouldn't be used for bottling because they either can't hold the necessary pressure, and/or they can't hold the proper seal needed for carbonation.