It depends on the color. Some need to be struck. Some don't. Some need higher temperatures to strike. Some need to be flame struck. Clear as mud now?
Seriously, though, most colors don't need to strike. Cobalts, blacks, greens, caramel/butterscotch, etc. don't need to be struck. The colors that really need to be struck are the Amber/Purple type colors. The best way to do that is to work them really hot so that they go almost completely clear. Then let them cool for 30-45 seconds out of the flame. Then introduce them into the back of the flame and they will strike.
It's best to try just some plain beads of one color first to get the hang of striking.