I don't even need to ask, is a nature thing girlfriend/wive suck cock for his man. Even my girl friend i hang out with automatically suck my friendly looking cock.
2 reasons if she don't suck you, either your cock is really peanut size (most likely not) or stinky cock (maybe).
Sit down with her and watch cock sucking jap porn, especially sucking cock series will help. Good luck and hope she suck your penis soon!!!