If the EMC of the kiln atmosphere is lower than the interior MC of the lumber, then drying should not stop. Either the kiln EMC is not what you believe it to be, or your MC sample is inaccurate. At 105F, drying will be very slow, so that might be part of the problem (not able to detect small decrease in MC).
Cutting the 10/4 lumber into parts only compounds the problem. Once cut, you have surfaces that are (reportedly) at 6-8% MC and other surfaces at 20%. The question is, what kiln EMC should you use? There is no good answer at that point.
Some questions: are you using samples? Do all your samples indicate that drying is not occuring? If some samples appear to be drying and others are not, probably need to cut intermediate sample in those boards not drying. Also, you need to check the kiln EMC with a hygrometer. We recommend the use of kiln sample boards to get the average moisture content for controlling kiln conditions.
Published schedules for 10/4 walnut for MC between 15 and 20% are pretty aggressive: 140F dry bulb, 90F wet bulb, resulting in a 2.9% EMC and 15% relative humidity. Your kiln conditions were not reported. Assuming the average moisture content is below 20%, I would suggest start by raising your dry bulb to speed the drying while maintaining an EMC of 4%. The 10/4 walnut drying step for MC between 20-25 should provide a safe margin: 130F dry bulb, 90F wet bulb, resulting in a 4% EMC and 22% relative humidity. For an extra margin of safety with this valuable stock, start with 120/90 before raising DB to 130/90.
I hope this helps, and good luck.