Your Fern looks neither dull or overgrown to me. It appears to be quite healthy, so I would be very careful about trying to "fix" it unnecessarily. The less you disturb the roots, the better the plant will be.
If there are spaces between the sides of the rootball and the inside of the pot, then it was probably repotted recently and maybe not done very well. Use a peat-based potting mix and push it down into the spaces along the sides. Don't worry about the space beneath the rootball. If there is such a space, the soil will find its way down there via gravity.
Keep the soil of your Fern damp, but not constantly wet. Fertilize sticks tend to create "hot spots" and can damage nearby roots so I suggest removing them.
