I'm having a problem installing a Tapo S515D Kit in a 4-way circuit.  I have installed the Master S515 in one end to replace a dumb 3-way switch and I installed the Slave S515 in the other end of the circuit to replace another dumb 3-way switch.  Both Tapo 3-way switches at each end of the circuit work perfectly, but the dumb 4-way switch in the middle is not functional.  I can control the light perfectly from either S515 on the end points and that works perfectly as you would expect in a 3-way circuit.  However, when I attempt switch the light on or off from the dumb 4-way switch in the center of the circcuit, the light just flickers and doesn't turn on or off.  

I must have a wiring error somewhere but I'm not sure what the problem might be.  I did not touch the wiring to the 4-way switch in the middle and just connected the travelers to the two 3-way end-point switches.  Does the order of the travelers on either of the 3-way end points make any difference in a 4-way circuit?  I know it doesn't make any differernce in a 3-way circuit.  Any help or advice will be appreciated.