The AHC linkage may be sticky. Verify that the cable/linkage moves freely, and the pot arm turns freely as the head is raised/lowered in the float range.
Test your pot: Back probe the B wire at the 3 pin connection. (use a paperclip to get into the terminal). Measure to a frame ground. With the head fully planted on the ground, you should have 3V DC. With the head raised off the ground, it should be close to 5V. Tnhis is the AHC signal your combine expects.
If you have the above, great, the problem may be in the combine. If not, measure the supply voltage (unplug header). Measure A to C on the round 3 pin - should be 8V.
If you have the 8V supply, but do not have the signal voltages above, either adjust linkages until you do, or replace the pot.