Are you sure you’re putting the correct strings on? I once got a call from a woman who said she had replace the wire F string 3 times and it kept breaking. I asked her to be more specific. Which F string, starting from the first wire string(G5). She said it was the second string and she was putting a 6th octave F string on there. “That’s 5th octave F, not 6th octave” I said. If you’re putting the wrong octave string on there, particularly a lower tension string, it might last for a while but then break.