Darting some creatures is just plain tough…Increasing you account level to boost the amount of DNA you receive per direct hit is always a plus, but it does take time.
One suggestion that I might have is to go out of your way to dart Commons that present similar darting patterns to the creatures you struggle the most with–i.e. target placement on Tarbosaurus and Allosaurus are similar to T-Rex, targets on Velociraptor are similar to Pyroraptor, etc. Obviously the target placement on Commons isn’t an exact representation of what you’ll be facing, but it’ll get you used to how the model moves on the map, how to deal with the creature taking sharp turns, etc.
Otherwise, just gotta keep practicing and darting those dinos! You’ll get those big DNA numbers eventually.