Going through the acoup blog as well as the AskHistorians sources, Sparta's win/loss ratio is just around 0.5. They won as often as they lost battles. Indeed, some of their major victories required Persian/Macedonian help or were actually accomplished by people from outside the agoge. And you forget that while they were the largest power on their patch of Greece, they were only hegemonic over Messania, no other area. Again, as the AskHistorians link points out, Sparta was much larger than the other city-states of its time at the point from where we have evidence. That Sparta was unable to do much from this position is fairly telling about their lack of skill in much other than hoplite fighting.
In addition, go through the series to understand the makeup of Spartan society. It's easy to have strong, well-fed warriors when more than 80% of your population is chattel slavery (read helots) who were thought to be badly treated even by the standards of the Ancient Greeks (which, as you can very well imagine, were not very high. Slaves weren't treated well by those societies at all). It's easy to lionise Sparta when you become acquainted with the Theme Park version of it. It's hard to stomach the actual reality of what we do know about Sparta.
I would honestly expect this of any non-expansionist state (and on longer timescales, expansionist as well), no matter how excellent their military. An army will attack only if they think they have a (good) chance of winning, meaning battles are likely to be at least somewhat even, or at the attacker's advantage.