There are 3 turnbuckles in the cable control run. In order to achieve the correct tension, I adjust the turnbuckles to maximum. I am not able to achieve the tension required. Why has this happened and what must I do to remedy the situation?

There could be too much slack in the cable and the turnbuckle bottoms out, the end connectors are slipping or you are stretching the cables beyond their yield strength.

Something may have bent?