Like crooketarrow said, natural shafting breaks all the time. If while straightening I hear a "tick" I go ahead and break that shaft and put it in the kindling pile.
You can tell when a shaft is hot enough because it becomes maliable where you've heated it. If it resists bending, heat it more. Also, straightening natural shafting(hardwood shoots and cane) is not something you do in one sitting. It is a process that may have to be done over and over again. And you should not try to straighten every crook in one session. Start with the worst bends first, heat and straighten a few of them and set the shaft aside to cool completely before the next session. I will work on straightening a few arrows at a time. After working on the worst bends I move on the the next shaft and so on until all the shafts have had their first straightening. I then set them aside on a flat surface until tomorrow. If you try to straighten too much on one shaft you will fine you bend the still warm repairs back to where they were. With any primitive technology, whether it be building bows, arrows, knapped heads or whatever, patience is your best tool!