Her clitoris would receive stimulation in cowgirl and missionary that it would not receive in doggy, so using your fingers or a toy or getting her to do it herself should help with her cumming when you are fucking her in doggy. Her bull having a larger cock than you, where penetraration is deeper in doggy, may be the reason she can cum with him fucking her in that position without having to stimulate her clitoris. You could always try a penis extention and see if that works.