The Slab is a tricky barrier. It can be broken only when chips fall down on it from at least three cells height directly from the column above, not from neighboring columns. In most cases a match of three vertical chips works to break the Slab, but not always. Here are samples of situations where slab will not be broken:

1 - If there is an obstacle above the Slab, it can prevent chips from falling on it from the required height. The chips fall not directly from the column above but diagonally from neighboring columns in such cases. The Slab can't be broken in this case. You need to remove that obstacle first.

See sample showing why it happens and how to destroy it: 

 2 - If there is an empty cell above the Slab, it prevents chips from falling on it from the required height too. An empty cell is considered as an obstacle for the Slab in this case. You need to fill the empty cell with a chip first. See if there are other obstacles that do not let the chips fill the empty cell. You need to remove them.

See sample showing why it happens and how to destroy it:

3 - A match of four or more vertical chips on the edge of the board can make chips fall diagonally from neighboring columns. As a result, the Slab can't be broken. To ensure the chips drop straight from the column above, make a match of only three vertical chips.

See sample showing why it happens and how to destroy it: 

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