New sorbus autumn spire flanrock tree, delayed bud opening

Hi, I've got a new sorbus which on the face of it appears to be growing quite well, appears to be getting what it needs. However in the middle of the main stem there are 3  buds yet to break? I have leaves and flowers above and below! Not planted over the graft joint, no problem with upward growth. Just looks weird. Any ideas ?  Anything I can do to encourage  mid section. Thanks