When you are only able to melt the wax near the wick, it leaves a ring of hard wax on the outer edges and that's not very nice. This is called tunnelling.
The only thing you have to do is covering the top of your candle with foil. Afterwards, cut a hole in the centre where the wick is. It encourages the hard outer wax to soften and melt evenly.
Burn your candle for several hours with the foil lid to ensure the whole top layer is liquid.