Sex position: Reverse Cowgirl

Me and my boyfriend have been dating 11 months, and have been sexually active a year yet we are to try reverse cow girl.
We have tried 3 times, but we can never get him inside me. I'm not sure if I'm sat incorrectly or something. Any advice girls?
Also, in normal cowgirl how do you move? I always lean forward, but I feel like I'm not doing it right or moving at all. Is it better to sit up straight? Eeeeek. 
Please help ☺️x

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