Pixhawk controller won't pick up every message

Question

I have been trying to inject raw UBX data which I gather from UBXReader library into my Pixhawk. For this, I use a GPS module to extract UBX data and a serial to USB converter to stream data into my Pixhawk. Here is what my setup looks like:

Using my other USB port, I gather GPS data and try to stream it into pixhawk as seen above. For this task, I use python.

from serial import Serial
from pyubx2 import UBXReader
stream = Serial('/dev/ttyUSB0', 38400, timeout=3)
stream2 = Serial('/dev/ttyUSB1', 38400, timeout=3)
while 1:
    ubr = UBXReader(stream)
    (raw_data, parsed_data) = ubr.read()
    output = parsed_data.serialize()
    stream2.write(output)

From MAVLink, I can see location and altitude data but I fail to stream HDOP and VDOP messages into my Pixhawk. What might be causing this and how should I proceed on fixing it?