Please stop ruining this part:
I do not know what has possessed you to alter the underside of the arch so that it no longer clutches bricks, but I have found myself overlooking any and all sets that contain high numbers of its inclusion (2x) because it is inferior to a part that once looked like it, yet did the job better. Why the change? Doorway and window arches still clutch bricks inside the arch, so why not the half-arch?
Please destroy your current mold and cast it over again, because it appears as though you have goofed.
A Shrinking Forest