Can't Place Pillars on a Certain Block

For some reason, sometimes when I try to place a pillar of the custom wall, 1 random block won’t let me and I don’t know why. Like all of the other blocks around it will let me, but that tiny 1x1 block wont let me place a pillar on it. Any help?

would you be able to provide a picture?

I deleted the world with the bug because I wasn’t happy with that town, but if it pops up again I’ll take a picture.