Those clips look abused, I've never had one break or lose tension. You must be opening them up too much and bending them.
You can take a new McMahon McRosco McWhatever clip out of the package, and squeeze it. Then take one off a handle or set of lines that's actually been used frequently, and give it a squeeze. You can feel that the used clip is softer after having been squeezed several times. They always loosen up with age. In normal use, the squeeze open action bends the clip. It's just brass, and eventually it'll break just like bending a paperclip back and forth. The point I was attempting to illustrate, is that eventually that bending will effect the brass. Fortunately, there are plenty of warning signs before they fail, like most of the chrome wearing off, no longer returning to the fully closed position, and a fairly obvious loss of spring tension. It takes a lot of use, but they do eventually fail.
I don't know that a hard stainless clip would last as long, or longer, but they have enough other benefits, and they're cheap enough.