No picture in the scope_sink of grc

hello all:
I recently make a graph in the grc. I use the signal_source
to generate a cosine signal, after the signal through the Gmsk
modulation block, I add a noise_source to the signal,then put it into
the Gmsk demodulation.but when the amplutide of the noise is higher then
0.18,I can not see anything in the scope_sink. Can anyone tell me why
this happened?


you very much.