There are some reasons for this.
One reason could be your aren't taking very good care of your hair. If your ends are damaged and you style your hair daily and want to grow it, there's a good chance it's breaking off gradually over time.
Give it a break from colouring and styling and nurture it with some treatments. Get regular haircuts and you will find that it will not only grow longer but will have thickness to it also.
Another reason could be that your hair is at its longest already. It's not something I like to tell people but it's the truth. Hair only grows to a certain length. To what degree of length that is varies with each person. And no amount of TLC will help.
Still don't understand?
All hair grows in cycles. Eyelashes, and eyebrows grow for 1 to 2 months. Arm and leg hair, 1 to 4 months. Generally the hair on your head has a cycle between 2 to 7 years.
This isn't to say it all falls out at once and starts again at once. If that were the case we would all be bald and start from scratch many times in our life. Each follicle on our head is in a different stage of this process you see. When it gets to the end of its cycle it stops growing and sits in the scalp for up to 3 months until you comb it or disturb it enough times for it to come out. This is why hairdressers say that generally we all lose 100 hair follicles a day. Each person is different though. If you have more hair than the average person you may lose a lot more.
So if you are having the latter of these problems then embrace your hairs length! Or get extensions