-He's barely even trained as a shinigami.
-Hasnt mastered his bankai yet (or used it to his full potential).
-He has practically no knowledge of his enemies.
-Yet he still invades their homeworld fighting battle after battle with little rest using a power he's unfamiliar with (somewhat)
-Has many issues toubling him.(poor mental state)
And we want him to master his powers and defeat all of his opponents too? Grimmjoww was enough. But making such a huge leap without a plausible explaination imo would be foolish. Ichigo has to struggle in every battle he has. He has to also receive help from his allies, because his opponents usually have huge advantages in which defeating them without some form ofhelp would be fluke-ish imo.
In conclusion, Ichigo is a character who is constantly discovering his new powers. He has virtually no time to train, and under the circumstances, is highly disadvantaged with an unfit-for-battle mental state to boot. You cant expect his ventures to always be successful.