As has been pointed out earlier the example of Xander is equally highly trained.
Two things:
The episode where he has "training" is a quarter of the way through the second season, so he's got quite a while without it.
That "training" only gets brought up a couple of times. He spends most of the series bumbling along like every other Joe, instead of like a Navy Seal or something. It is rather like Kitty Pryde's ninja training - only pulled out when they can't figure another way to deal with an issue.