[Discuss-gnuradio] UHD USRP sink does not react to gain changes (gr-ieee802-11: between two usrp devices)

Dear All,

I need your help. May be any of you had such a problem before:

I use 'IEEE 802.11 a/g/p OFDM Transceiver' project, Ubuntu 13.04, GNURADIO 3.7 and (USRP N200 + XCVR 2450 + VERT2450 antenna).
I start the transmission using the first laptop and USRP N200 device by running the ./ofdm_tx.py.
Meanwhile I use the second laptop and USRP N200 device as the receiver by running the ./ofdm_rx.py. The error rate is very high.

The problem is that when I change gain in the USRP sink nothing changes (i tested it using 'wxgui_fftsink'). 
(I think it is a software problem since I tested it with another project. There it changes when I change gain.)

Do you know what can cause this?
Why transmission side (USRP sink) does not change according to the gain at all?

Thanks in advance!
--
NE