Simple? MPX Profi 4000 ?
Do you mean the servo stops moving at about half stick? The most common cause of that is the servo hitting its mechanical end stop because you have adjusted the control or servo centre to way off normal. If you have autotrim enabled you could have cumulatievly added trim until without realising it the servo is way off centre and near its end stop. Another possibility is you have multi point curves and you have made a S shaped curve so that all points beyond mid stick are the same value.
Check that menu 4 Tx mode is PPM9 and neutral pnt = other. In menu 1 make sure that the control centre is at 0 and travels are at 100% both ways, trim centre is at or close to 0. Ensure autotrim is off. Then check servo centre and limit for 0 and 100% respectively, then travel/curve for a straight line. If you need 52% travel, then adjust it in this menu, not in the control travels menu.
Harry