No idea. What is the device you’re trying to write to? Is it a fob or implant or card or ring or something else? The only thing I can think of is a coupling issue that is good enough to read but not reliably write.
Coupling can also be “overpowered” in some cases where timings are off. Try putting some space between the device and LF antenna… like 5mm increments.
Maybe post pictures of the target device on the proxmark3. That would also help answer what kind of proxmark are you using.