No sap coming from holes after drilling

This is my first year tapping and have a question about sap flow. I have a large black walnut grove on my property and decided to try tapping some of the trees this year. I tapped ten trees a couple days ago and when I drilled the holes on eight of them the sap began flowing immediately, however the holes on two of the trees were completely dry after drilling. I tried drilling an additional hole in both trees but those were dry as well. When I collected sap yesterday I looked at the holes I had drilled on both trees and they were still dry... no sap dripping at all. Both trees are 14" in diameter. I believe all of the trees had leaves last year so am not sure why there is no sap. Any thoughts would be greatly appreciated!

Were the two dry holes on the north side of the trees perhaps? Those will take a bit longer to thaw...still fairly early in the season, especially for walnut.

All of the trees were drilled on the south side. I checked the holes again today but they are still dry so will keep an eye on them. Hopefully the sap will begin flowing once it warms up a bit more. I collected 3 gallons of sap today from the other eight trees (yesterday I got 2 1/2 gallons). Thanks very much for your reply!