Spruce Slabs Disapearing!

So I Was Playing On My Survival World In MCPC And then I Went Into My House And Some of My Slabs Has Disappeared I Assumed It Hadn't Saved Or Something So I Replaced The Floor And Then I Used An AFK Fish Farm Over The Night And I Came Back In The Morning And Went Into My House Again And They Had Disappeared Again!!!! Please Fix This, From Katsy_Kan