You have probably already tried these things, but in case you haven't:
If you have predators in the pond, the minnows likely will be hugging the bank for protection (assuming you do not have a lot of weed cover for them within the pond). Try putting the trap so the thickest center portion is just barely covered with water. Really shallow near the shore.
Keep the trap parallel to the shore line. Minnows move along the shore line as they feed. If the trap is not oriented with the openings in line with the way they are swimming, they are less likely to go in.
I've had variable success with traps. Some time periods catching very few FHM, other times catching lots of them. But not catching any over a period of time, that is new to me.
Defective minnows?????
