I only just found this out the other day myself - If you just do a straight save in the Expressions library it is actually saving the head morphs along with the expression you have made . To ONLY save the expression you need to click on the SUB-SETS button in the save window which appears after you click + , then the universe box in the drop-down which has now opened. That will turn ALL head morphs off. Then turn on 'head' and then any of the expression morphs you want to keep. Don't forget to turn 'head' back on in the list.... I did for a day and still couldn't understand why my expressions weren't saving.

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-Monty Python