Just to be clear, you hit the button and it moves 6", then stops. Hit the button again and it moves 6 MORE inches, right? That is the motor sensing a too-high current draw and stopping (current limiting). That is nearly always caused by either an obstruction of the slide (too hard for the motor to move easily) or low battery voltage. You can have low voltage to the slide motor even if the battery itself is OK. A poor connection, power or ground, would cause that.