Something strange is happening on my map.
The Mygeeto platforms (myg1_bldg_large_platform_a, b, b_collisions) have no "substance", everyone falls through them, I can shoot through them. The upraised edge is fine, I can walk and shoot on it just fine, but the main part has issues.
Any possible way to fix this?

You need to put the B_collisions on the rest of the platform, overlapping it, and at the same height as it (Basically, put B_collisions right on top of it, and at the same height).