Players getting stuck in slabs?

In certain cases...:

Is it WorldEdit?
Sometimes when you use WorldEdit to place half-blocks (half vectors) they're not rendered fully by the plugin. Perhaps digging them up and replacing them by hand can help.

Is it Marriage?
In some configurations, server administrators can use blocks as "pedestals" to show off two players (that are married). Sometimes, when the incorrect configuration is used, the server may get confused and think it's using the wrong block. (resulting in the passthrough of the slab).

Is it your Permissions manager?
Sometimes when a player (or player group) doesn't have proper permissions to move into certain areas, pass-through of blocks may be inflicted by a very confused Java Event Handler. Try using WorldGuard to allow the passthrough of these areas to settle Java's confusion.

Let me know if any of these helped.

I'm no help with your issue because you don't have the plugin I had the issue with.
But when NoCheatPlus caused this issue on my server. Maybe you secretly have an anticheat that's grabbing your feet and you don't know it