this is one of the mysteries of the TD build. there are a couple of other threads that address this. there is something about the geometry of the gallery flat relative to the sides that prevents it from going into its location without a lot of force.
when I was dry fitting it, ONE time it slipped into place but I couldn’t reproduce that phenomenon. so, I sanded about 1/16 off each side, which permitted me to get it into place, but then there was a 1/16″ gap on both sides!! go figure!
the good news is that after applying the fillets, top and bottom, the oversized gap was filled and all was well.
ken