Actor falls through tiles - tile edit already checked

Hi all,

I am having difficulty. I have an actor dropping onto tiles, running, before they are met with the end of the tiles. Instead of falling through, the actor seemingly hovers, standing on literally nothing. Another few steps along sees the character back on the same tile, only it is here that the character falls through the tile.

I have attached a photo to better explain the issue.

Can anyone explain what is causing this and tell me how to fix it?