the gearbox went on monday and have had to have a second hand one fitted along with a new clutch and plate but now my clutch is very high and makes like a shuddering when setting off

somebody said the thrust release bearing may be in wrong and not sat right but i saw the person fitting and the arm that holds the bearing on looked to be on correct and moving freely before fitting the box back on ( had to help lift the box from above )
All drives fine with no slipping just the shudder when setting off and the high bite point.
Any ideas guys?