While a player may have a technically good shot, a drop off in any one of these three areas will decrease shooting percentages.
Consistency comes from repitition.
I'm presuming you spend an adequate amount of time free throw shooting in practice. So if that's the case then the problem lies in the other two areas, concentration and confidence. Free throw shooting becomes mental. Team free throw shooting can drop off as the result of the teams consciousness about poor foul shooting as a group. With each miss, the team or individuals put more pressure on themselves and don't shoot relaxed and confidently.