Check your angle of entry. When mounting a bitch, thrusting "straight in" (with your penis oriented more-or-less parallel to the floor/the bitch's back) is all but guaranteed to fail. You MUST angle upward, usually close to, sometimes more than, 45 degrees upward. A straight-in approach is pretty well guaranteed to fail to achieve entry - You're either going to end up with your penis trying to enter the clitoral fossa - a short, dead-end pouch below the actual vaginal entrance that's *FAR* too small for even the tiniest human to enter, and has nowhere near enough depth to receive a penis - or bash into the back side of the pubic bone. Either way, the result will be failure to enter the vagina, likely with your penis popping out of her vulva and sliding under her belly, or getting trapped under her tail.
An upward entry angle - as noted, 45 degrees, sometimes even more, is required to bypass the clitoral fossa, and direct the penis so that it skims past the pubic bone and approaches the actual entrance to the vagina, allowing actual entry. Be aware that once you get that far, there's still another obstacle to full penetration - at the entrance to the vagina itself, a bitch has a muscular ring that, like her breathing, is only partly under her control - she can relax it to a degree, and she's likely to clamp tight if she's hurting or afraid, but she can't choose to FULLY relax it - that usually only happens when she's delivering puppies, and is mainly controlled by hormones softening and relaxing the tissues, and the force being put on it by her contractions trying to drive a puppy through it. This ring of muscle will very likely require "working your way in" or "forcing past" it ("force" in the meaning of "OK, it's a tight fit, so I have to press harder to make it happen," as opposed to "OK, bitch, you're gonna take my cock whether you want to or not!") to achieve actual entry into her vagina - More foreplay may help to relax that ring, but it's not guaranteed, and in some bitches, it may never be possible to insert a human penis into her vagina, no matter how willing/cooperative she is - at least, prior to her having puppies (and even then, it might never be possible to get a human penis inside her).
Yes, I already hear somebody saying "but the dog I let fuck her has a dick the size of an elephant, and he got in just fine!" First, no he's not quite as big as an elephant, and second, no matter how big he is when he's fully erect, when he mounts and enters her vagina, he does it while VERY slim - barely larger than the bone inside his penis, which is variable in size, but is only rarely much thicker than a pencil - often, it's thinner than that - and then swells to full size once he has entered the vagina. We humans, on the other hand, have to be "fully inflated" (and thus, MUCH (anywhere from 3-6 times or even more) thicker than any dog penis is at the moment of entry) to achieve penetration into pretty much anything, including human females or even critters as large as a mare or bigger. Once past that ring, you'll notice a clear (although I find it very difficult to explain it) change in the texture as you enter the actual vagina and eventually achieve full penetration.
Also, keep in mind that once you've achieved full penetration, her instincts are going to drive her to simply brace herself and wait until you withdraw - once penetration is complete, a male dog is typically locked in place. My point here being that full-length strokes are PROBABLY not going to be appreciated by the majority of bitches, while short strokes and "wiggling" may greatly enhance her enjoyment of the mating. Using fingers externally to stimulate her clitoris may, depending on the dog, enhance her pleasure, and in SOME cases, cause her to start humping in nearly the same way a males does as he's mounting. Count yourself lucky if you get that reaction - it's QUITE the pleasant experience!
