I often exercise and my muscles are growing well. But my strength is not increasing. Why am I average though I work hard enough?

If you worked hard enough to increase your strength, then your strength would be increasing.

Every body is different; you can't train under the same loads as somebody else and expect the same results.

So even if you are training as hard as somebody else whose strength improves, you can still find that yours will not.

If you train as hard as you can, you will probably find that you have a lot less endurance (for lifting those loads) than you usually manage to display. Don't worry about that. as your body adapts to lifting close to its limit, the number of lifts you can do will increase as well as the load you can train against.