Only sterling silver crimp tubes work with the crimper. If your using base metal crimps you have to just flatten it. Half the time, with base metal crimps, it comes undone.
If you can get your string to thread through the crimp and a few extra beads if the crimp does come undone, you have that extra tail to re-crimp it.
Double check crimps, before and after you crimp.
Also they make crimp covers, and you just put them over the crimps (using the front part of the crimper, gently squeezing it together)
Hope that helps