That would be a store, and in particular!.. a salesperson to avoid next time.
At f/1.4 they can be dreamy, but with copious amounts of USM they're still sharp lenses.
At f/2 you should have razor sharp images irrespective of what focus method you're using.
They are renowned for focusing issues as Andrew pointed out.
Does your camera have Live view?
is so, and on a tripod, use Lv mode and see if you can get better focus in that AF mode too.
Do you have AF fine tune adjustment in your camera as well. try that as well.
At least with armed with more knowledge, you'll be better able to tackle any problem.
Mine is 90% good(accurate) but sometimes it does want to miss the target too, even if I refocus properly, and I AF again, it wants to defocus instead of holding the correctly set focus adjustment I'd just made. Then a few moments later, on a different subject it'll work perfectly!
maybe it is self aware? Maybe it knows it's a portrait lens and wants to produce softly focused dreamy looking portraits of a flattering nature... despite best efforts to the contrary