The types of explosives are linked to the size of the matches, starting with dynamite on a match 5, bomb 6, of type match dynamite on 7 and above. So if the play area gets clogged with Stones, then it's hard to match. My strategy tip is to use the Reset button to clear out the grid, and start a fresh, get some new explosives form easier matches, then blow up more stones! :).
Hope that helps you on the way to the Danger Mine! :D