Right I dont know wheather you will find this weird or not but my girlfriend had this problem no matter how hard she tried when masturbating on her own she couldn't orgasm.
Have you tried a lot of different techniques? Right now for the weird part.
When masturbating with your finger, insert it and then go up and push away from your body. Effectively stroking the top of your vagina. My girlfriend can finally orgasm while masturbating using this method.