Also why are there spinners like Tahir and Narine who are very good in the shorter form of the game but not made it in Test matches? Is it that they are not good tactically? Or they are not consistent bowling in the right areas?
I'd argue the latter; in long-form cricket you can wait for the bad ball and capitalise, whereas in T20 the batsman is always going at the bowler. Both Narine and Tahir have variation that, on the majority of Test tracks, isn't likely to get you out if you're playing defensively -- whereas in T20 that same variation gets wickets, because the batsman is always looking to score and hit big shots. Easier to adjust to a ball turning the other way when you're defending, compared to when you're trying to smack the ball over extra cover for six.
With Narine in particular, there's also the flight issue. Narine's natural length is hard to get away in T20, especially with his pace and trajectory. Thus batsmen have to take more risks, and make more mistakes. In Tests, his tendency to bowl flat (a result of technical limitations) stops him from beating guys in the air compared to more traditional offies like Swann. On a helpful deck he'll be seriously dangerous, because he bowls quickly, flat, and will still get dramatic sidespin. On less turning tracks, not so much.
Rashid gets nice drift, so I don't see him falling in the Narine camp, but it does seem to me that he might bowl a touch slow for Test cricket -- he's loopy, so you can play him off the pitch and watch for the variation. Plus he can almost turn the ball too far at times, beating the edge rather than catching it.