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Make sure the hearth bricks are level. As tempting as it might be, the ceramic board is uniformly flat but your bricks may not be perfect. I played for an hour sorting bricks so they would fit with little or no gap. You'll find some narrow at one end or thicker or high on one end. It's just the way they are made. C5's suggestion to put down a thin bed of fire clay (and silica sand) to level the bricks is a good idea.
Why not mortar the hearth bricks? Mortar invariably gets between the bricks even if you are careful, and creates slight a gap. Mortared hearth bricks are not easy to replace if you ever need to do it. Then again, you may find the replacement brick won't even wedge itself into place because its neighbor won't budge.