This one really had me going too. I even bought another IR543, but it exhibited the same problem. I've read numerous forums concerning this issue, but no one seemed to really have a definitive answer as to how to fix the problem. After much experimentation, I think I've found the solution.

The problem seems to be the amount of ambient infrared light reaching the IR543 sensor. The device is actually being "locked up" by this ambient infrared light. Since we can't see infrared, it's impossible to detect it's origin.

The solution was to limit the amount of ambient infrared light reaching the sensor. I first tried using a piece of opaque tape with a small hole in the middle of it in front of the sensor. Both of my IR543 units functioned perfectly after this mod.

I've produced a set of two black-on-clear stick-on P-touch labels with different hole sizes that I can send to anyone who is experiencing this problem to modify their unit.

Let me know if you need one.