interrupt pin is not rise once for a while

I am using TRK1000 with “ex_01a_simple_tx” software transmitting every 100 ms
Second TRK1000 board using “ex_02a_simple_rx” software.
Everything fine when using poling for receive.
Now I set interrupt pin “dwt_setinterrupt( DWT_INT_RFCG , 1)”.
It is not rise once for a while (after few receives ~10 ~100 seconds), but I am receiving this frame.
I am using this pin as a flag only, so receiver is working and interrupt pin will rise for a next receive.
I am using scope with one channel in receiver and the other one on interrupt pin

Any suggestion ?