True.
The reason, Sparta could not hold onto its victory has mainly two reasons:
1) The Spartan armies was desperately lacking manpower. The elite fighting class of the Spartiates had almost perished. Their defeat at Leuktra destroyed their fame of being unbeatable thus stripping them of their big joker in Classical Greek warfare: the morale bonus. Spartiates considered themselves invincible in "equal" combat and so did the Hellenic world. In a classical hoplite battle, where morale was a considerable factor, this was quite an advantage. Leuktra put an end to that.
2) The government system and institutions of Sparta (their constitution so to speak) was not able to deal with rapid expansion. It was a defensive constitution, meant for a quite hermetic and static state. Sparta tried to establish similar government systems in subdued poleis which would of course inevitably fail because the hermetic and strict organization of Sparta's mostly agricultural state could not work in more dynamic, urban-oriented societies like Athens. This created a lot of resistance and unrest which in turn had to be opressed with military force which of course became harder and harder to achieve due to 1).