This doesn't seem as uncommon as I thought it was.
I have only had one partner, out of many, that has made me cum from hand jobs or oral and who I felt comfortable masturbating in front of and it took a long time of knowing her before I could achieve these.
The 'secret' with her seemed to be that she was really, really enjoying it and when I eventually managed it (from a hj, during which she literally begged me to cum) from the sounds she made as it happened it sounded as if she was cumming as well. Her 'praise' of my cocks actions as 'it' happened and the amount produced helped on subsequent occasions.
With others it almost seemed that they were carrying out 'duty actions', which I hated (I don't want anything sexual to be done from 'duty'), or were 'expected' of me and I wont/can't 'perform' to 'order'.