The two pieces of the half-lap joint need to be flush at the top and bottom. Sometimes sawdust in your work area can lift the board you're cutting off the surface of the saw table. This will give you a bad half-lip joint. To solve this problem, shim out the fence so the front table leaves a gap for sawdust to fall into, away from the table.
Draw a line on the sacrificial fence to indicate where the fence meets the table, then remove the fence.
Pin two spacers to the sacrificial fence just below the line you have drawn. Don't go above the line, or the spaces will impede the piece you're cutting.
Drop the fence back into place and you'll have a gap for sawdust to fall into.
Another problem when trying to cut a board is when it's warped. You'll need to hold the board down securely when cutting. To do this, use a brad nailer to attach blocks to the sacrificial fence just above the height of the board. Place the board securely under the blocks, then make the cut.
Use brads that are just long enough to hold the block in place temporarily. You'll want to remove the blocks later to make other cuts.