Even when it is practiced, accomplishing shamata is rare. One of the very common problems is that people try too hard. Both Tibetans and Westerners could learn a lot about relaxing more deeply and letting the stability arise from that relaxation. Although it is mentioned in the texts, the Tibetans sometimes do not emphasize this point, but they do emphasize tight attention, not letting your object drop for even a second. If you are coming from a very serene space, and your mind is already very spacious, then that is probably good advice. But otherwise, such attention can be a big problem. You can exhaust yourself and cause nervous fatigue, and if you push it, you can really do yourself some damage.
From The Four Immeasurables: Practices to Open the Heart by B. Alan Wallace, page 83