Here’s an easy-ish way to tame these guys if you don’t have access to tranq arrows or if you do but your bow won’t accept them (happens to me every time xD) find a rock to climb on top by their tail or build a thatch building, one base, three walls high and a roof on top at the opposite end as the ladder (the ladder shall face the same way the foundation is facing in comparison to the roof) and climb onto their backs. This way they can’t push you (they are trying to be friendly and they think that pushing you is a game. It’s technically more of a “nuzzle”, though they tend to underestimate their own strength). Just be sure you have a boatload of berries on your hands and you can feed it until it’s tamed whilst getting a ride.
Don't bother knocking it out. Jist get your back against a stone and make the diplo come right above you. The gap between the first two feedings will be huge so be prepared to sit in front of your screen for a while
When taming to make it easier stand against a cliff or a good sized rock. I tamed one for my brother and it was a piece of cake with that technique. Also if you find that the place you find the diplodocus isn’t safe just get near it and lead it away. They won’t hurt you they just like to swat you around like they are playing a game of soccer.