I’m not sure what equipment is involved exactly (and you don’t need to share), but there is a big difference in angles depending on which hole you’re entering. If the top is entering a vaginal canal, you’re going to need to arch your back quite a bit, which means stretching.

Daily stretches are best. As weird as it may feel practicing for this, you should do other daily stretches anyway and just incorporate this in.

Also, there’s a good chance, depending on your partner’s height relative to yours, that they are simply too low (or too high) to get the angle right. They might want to experiment with what they are kneeling on to make it easier for both of you.