it can be very whimsical to get SFM to place down skulls, I suggest trying all kinds in creative and see which works.
for one I always make sure there's a block that the block gate can place the skulls against, so make sure that there are blocks placed 2 blocks away from the block gate.
another is to try to switch between normal block gates and block gates placed inside cable clusters, for some reason that also affects whether a skull gets placed or not
it's just a whole lot of testing to find out which will work, a build I did in a creative world which did work would still end up not working in my survival world