If you're using strippers that have sharp blades on them (little notches that fit your wire sizes perhaps), you are cutting not only the coating of the wire but the wire too. This means you have penetrated ever so slightly the wire that over time can fatigue at that cut and eventually break or decrease the carrying capacity or both. Look at your wire and see if there isn’t a nick where the coating was stripped. That’s where it breaks. A thermal wire stripper is a good way to prevent these problems:

Downside? High cost … $300 or thereabouts. Ebay has ‘em for way less.

Jim