I went back and studied the picture for your spool with the cutter string on it. The starting tip of the string is inserted into the wrong hole.
If you look closely, there appear to be a small piece of old red cutting string is in one of the pyramid shapes hole down inside. That suggests to me that it where the end of the string should be inserted then run to the outside and wound around the spool.
Also note the notches cut in the out edge of the spool, they are used to hold a short section at the final wound end of the string, so the spool can be reinserted.
Once inserted on the drive section gently tug on the cutter string end to pull it out of the groove and enable it to bump feed.
Hope this helps you.