I am glad to hear you are enjoying oolongs from Taiwan!
A couple thoughts in regards to your Ali Shan (spelling tents to vary in America) experience. Ali Shan (like Li Shan, and other high mt green oolongs) tend to lose flavor as they get older and are exposed to oxygen. Roasted oolongs on the other hand retain much of their flavor and can even be aged to achieve a different flavor. Generally speaking, if the tea you are buying is lower in price you are probably better off buying a heavier roasted oolong.
Also, it really does depend where you buy your tea. Some stores really just do not know when their tea was harvested or from what region. I really only buy more expensive teas when i know exactly where they are from, when they were harvested, and from people/stores that specialize in such teas.
Finally though, it might be that Ali Shan really is not to your specific tastes. Many americans like the roasted oolongs better because they have a stronger flavor (which is also more familiar to them). This is not a bad thing, just a matter of taste.