Couple of check that you can do.
- Are the clamps closed properly around the wire?
- Check that same phase is measured with clamp and voltage measurement wire wired to HEM.
- Check that clamp direction is correct and same in all phases
- As far as I know HEM should be able to measure also negative wattage values-> production
- In which phase the solar panels are connected or is the inverter 3-phase model?