What drives the MCLR pin of the slaves? A PIC output?

I am asking, because the MCLR pin needs a level of 0,2*Vdd to see LOW (for the 16F177x series, but I guess it is the same for others).

It depends on output characteristics if you are within limits of the MCLR. So be sure that what you think as low is actually sensed as low on the MCLR pins.

Ioannis