Free tiles
Most beginner mistakes start here: reaching for a tile that cannot move.
Two conditions
A tile is free when nothing lies on top of it and at least one side — left or right — is open. Both conditions are required; one is not enough.
Why a neighbour blocks you
Tiles stand shoulder to shoulder in a row. If there are tiles on both sides, the middle one cannot slide out even with clear space above it.
Checking quickly
Look at the surroundings rather than the tile: what is above, what is left, what is right. Turn on “Dim blocked tiles” and everything unavailable fades back at once.