I think he means "the first RP6 to the second and the second to the third."

I have done a similar scenario with 4 RM433s and 6 RP6s, and it worked perfectly. I can't help think that somewhere the wires are crossed from the RM433 to the RP6, or between RP6s. Because your description of the wiring is correct.

Just to recap: the 12v, ground and signal from the RM433 connect to the 12v, ground and signal "IN" of the first RP6. Then a ground and signal "IN" is paralleled from the first RP6 to the second RP6, and the same from the second to the third.

Are the signal and ground wires twisted together at the the first and second RP6s, or are they paralleled with connecting blocks or terminal strips?

Do all the connections make good contact with terminals at each device?