Under normal circumstances, the pieces that appear in the miniboard are not truly random. The game "helps" you by making a random number of pieces either A) match the one in the bottom row of the main board; or B) match the one to the left of the piece that is appearing. In general, this ensures that you will actually have playable "sets" which you can click.
After Level 20, this effect diminishes a bit, to make the game harder, but never under normal play will the pieces that show up will never be completely random.
Unless . . .
If you click the "Random mode" checkbox on Edit Options, Rows and Pieces tab, the game will no longer "help" you. The reason that I recommend this "for experts only" is that this mode makes the game quite a bit harder to play. But, if you want a little bit more challenge, go ahead and use this mode. Have fun!