We are trying to program our robot to lift with 4 motors but when it is programmed to the 6U/D button on the controller it will not function. When we press 6d it goes up but when we press 6U it does nothing.
You need an additional else between the two if statements. The general form is
if( vexRT[Btn6U] == 1 ) {
// run motors forward/up
}
else
if( vexRT[Btn6D] == 1 ) {
// run motors backwards/down
}
else {
// stop motors
}