The simple answer to your question is that a woody cutting will not root until the cut end forms a callus. This can be difficult with some plant material. May I suggest that for those hard to root shrubs/trees that you try my 'toothpick' method. It is easy. In August select those cuttings you wish to root. Now with a small, thin bladed knife or Exacto knife make one parallel cut all the way through the stem below a bud and insert a toothpick through the cut. Secure a marker on the branch so you can find it later. Walk away and forget about it until November. At that time the cutting will have callused on the plant. Remove the cutting below the toothpick and pull the toothpick out if possible or cut off the toothpick at either end close to the stem. Now the cutting should root in either a cold frame or greenhouse. I have even tucked them right next to a house foundation for the winter and found the cuttings rooted in the spring with no protection. By the way, the best method to try and root apples is to use the water sprouts as cutting material.

That sounds like a most interesting experiment!!!!! I will try this toothpick method of yours soonest.
Hmmm and water sprouts you say. I have pear trees that have been , ahum ... a bit neglected and have water sprouts a plenty. I'll see what they do with the toothpick method on their water sprouts.
THANKS for the great idea!!!
Mike