I was having the same problem so I figured the Sigma magnet must be stronger. So I did a test and discovered that the Cateye magnet was 2-3 time stronger than the Sigma. After some head scratching, I replaced the battery in the pickup and all was fine.
1) The longer/ lowered power Sigma magnet is more effective than the shorter, more powerful Cateye magnet. At least with the Sigma pickup it is.
2) This is the second time I've gotten a device with a stale battery (my Specialized computer came with a dead battery). Manufacturers must skimp and ship low quality magnets.
BTW, I really like the size and shape of the Sigma magnets. Seems like it's there for good once you torque down the set screw.