MCPE-13799Gaps in Slabs can't be Filled Unless Cursor is Pointed in Certain Place (See Image)
Update by [Mojang] Mega_Spud (Jay Wells)
Partially Fixed in 1.1.0.0 - Wooden Slabs (all variants) still affected though.
Slabs of the same kind can't be stacked unless they are placed directly against the other half slab.
The gap should be able to be filled no matter where the cursor is pointing, to make the 2 half slabs a solid block.
In other words, it should detect that you are trying to make a double slab.
Difficult to describe, hopefully the image will explain it better than I can 
[Mojang] Mega_Spud (Jay Wells)
2016-02-26, 08:37 PM
2021-01-15, 10:14 PM
2018-10-15, 11:22 AM
6
5
8137 / 106436
0.14.0 - 1.9.0
0.14.0, 0.15.0 Realms Alpha build 1, 0.15.0 Realms Alpha build 2, 0.14.2, 0.14.3, 0.15.0 Realms Alpha build 4, 0.15.0 Beta build 2, 0.15.0 Beta build 3, 0.15.1, 0.15.2, 0.15.6, 0.15.7, 0.15.90.7, 0.15.10, 0.15.90.8, 0.16.0, 0.16.1, 0.17.0.2, 0.16.2, 1.0.0.1, 1.0.0, 1.0.2, 1.0.3, 1.0.4.1, 1.0.4.11, 1.1.0.0, 1.0.6.52, 1.1.0.4, 1.1.0.8, 1.1.5.0, 1.2.0.22, 1.2.11.4, 1.2.13.6, 1.2.13.60, 1.2.16.3, 1.5.0.1, 1.8.1, 1.9.0