Hum.... I just try the 2401 on 4#6 and the v120d01. Both will bind and function without any issue..... Maybe the 2401 is defective? For the 4#3q double brushless, check if the motor is turning freely. And resolder all the wires to the tail motor. As sometime the join look ok but it is not transfering the electricity as it should.... For 2801 manual please refer to the following link. For the 4#3q and v120d01 make sure you switch the TX to 2401 mode.
http://walkera.com.cn/en1/fileDown.j...expe.pdf&la=en