Doggy style

I recently started having sex, and so I'm still learning. My partner and I have been trying doggy style, but whenever we try his penis can't stay in... Like the first thrust will be fine, he will be in and it feels great, but when he tries to keep thrusting he slips out, and it takes a solid 5-10 seconds to actually get back inside. Not sure why this would be happening... Height difference? Tightness? Not wet enough? Not positioning ourselves the right way? Please give some insight or tips 😊😊 thank you!!!