It's probably not you, more likely the pellets.
Every barrel is different and it's only by trying several different types in your barrel that you'll know what's best for your particular rifle.
If you shoot, say two five-shot groups with one type of pellet, then repeat with different pellet types, that will give you some idea of what each pellet will do.
But to be fair to the pellets, as said above, you should really clean the barrel, take 10-15 shots with Pellet X to re-lead the barrel , then shoot a couple of five-shot groups with Pellet X.
Then repeat the whole process for Pe