They could never really beat Athens decisively in the Peloponnesian Wars. I tend to think that they were all Brute no brain when it came for warefare. Lacking military strategy and tactics. I know the Spartans lacked a Navy while fighting the Peloponnesian Wars against athens. However I feel as if that proves my point even further. What of kinda idiot do you have to be to be a port city and not have a strong navy.
I highly recommend you check these posts by /u/Iphikrates, who's basically the patron god of Spartan history.
Is the military worship of the Spartans justified?
How does Sparta compare to Athens and Persia in the Fifth Century BCE?
Were the Spartans really all that great as warriors?
Why didn't Sparta enslave/destroy Athens after the Peloponnesean War?
Why did Persia side with the Spartans in the Peloponnesean War?