Stealing, as I understand it, requires your total including at least one worm to match exactly that of the topmost tile of an opponent for which you wish to acquire. The only time that a tile of the same or lesser value can be acquired from the main play area is when the total including at least one worm is the same or greater than the tile you wish to acquire.
e.g. opponents exposed tiles are 23, 27, 29. The lowest tiles available in the main playing area are 24, 26, 28, 30.
Your current total including one worm, 3 fives and one 3, is 23.
At this point you could:
- steal 23 from an opponent
- roll again, hoping for one and only one four to steal 27, two 2's to steal 27, three 2's to steal 29
- Anything in between would allow you to take lower tiles from the main board
- not rolling any 4's, 2's or 1's would mean you lose your top most tile from your playing area.