What scarlettariel said is right, especially her first point. If you are unhealthy and your body is not getting enough nutrition the first signs are your hair and skin, which can result in slower hair growth, as your body would be using the nutrients for vital organs.

Also try to cut back on heat, which causes breakage and will appear as if your hair is not growing. 

1) Trim you hair every three weeks to a month, depending on how much you use heated hair products (hair dryers, curling irons, straighteners, etc.) Not trimming damages your hair, because trimming gets rid of split ends and results in healthier hair altogether.

2) Drink a ton of water. The recommended amount is 8 glasses a day.

3) Onions. Eating onions helps you hair grow like crazy, no joke. My cousin eats like one a day and grew her hair from her chin to her elbow in like 8 months. It also helps to rub onion juice into you scalp, but that seems a little extreme.

4) Rinsing with vinegar. This doesn't exactly help hair growth, but it gets rid of product residue and makes hair healthier, and everyone knows healthy hair grows better.

       Keep in mind that hair only grows about 1/2 inch every month.

You actually need to trim your hair every 3 months to stimulate growth.