I had the same problem with the softserial RX not responding.
in the end the fix was quite easy: increase the buffer size for newSoftSerial.
in NewSoftSerial.h on line 38 a constant for the RX buffer is defined:
#define _NewSS_MAX_RX_BUFF 256 // RX buffer size
default size is 64. this is not enough. I increased it to 256 and it worked.
cheers