Clitoris is very sensitive. If treated harshly it will get numb. Direct stimilation is difficult. you may try female deer exercise. Try circulating your fingers around clitoris.Continue to search for stimulating clitoris, remembering not to do any thing aggresive directly against clit.
Your partner may be experiencing something called 'anorgasmia', which is the inability to achieve orgasm and lack of sensation in the clitoris. I had to look this info. up online, on a couple different websites. There is treatment for this condition, by way of your partner seeking treatment from a physician/ gynecologist. Two of the sites you may want to check out w/ your partner are; netdoctor and healthcaremagic . Once you are at the site, you can use the search bar to type in anorgasmia, to see if this compares to your partner's symptoms.
Good Luck