I have this same problem - but only when I use those 10kg plastic bags of 'House Coal' from convenience stores or DIY outlets. Really, the coal in them is awful stuff, and is almost impossible to light and to keep going. I don't think it's meant for open fires, but it's like the stuff we used to get for a closed-front stove we had years ago. Lately, I've been getting coal from a local coal merchant instead - big, shiny, brittle lumps in 50kg sacks. It burns beautifully, looks so much better, and is dead easy to light using only newspaper donuts.