Is this happening when you are solo or with a partner? Either way, it is not unusual and it comes right down to gravity. When your heart is at the same level as your penis it takes stronger blood flow to achieve and maintain a really firm erection. Your arousal level and elapsed time since last ejaculation come into play as well. And if you are on your back and she’s on top, say reverse cowgirl, things become even worse because her body
weight on your pelvis and/or thighs will decrease the blood flow where you need it most. Try standing on the floor. Slide her to the edge of the bed. From the standing position you can perform a modified missionary, doggy or piledriver. Since your body's upright, you’ll have maximum blood flow as well as full control of the pace. Admire the view and stimulate her clit and breasts. You’ll find you can last longer this way as well.
Practice your kegels and reverse kegels. This will improve your EQ and command over your erection greatly. I would recommend taking 25-50 mgs of Suhagra. You’ll be pleasantly surprised. (rock)