You shouldn't need to place the mouse on the stream. When you start WSJT-X and select a band, as long as there are stations transmitting, and your radio is passing audio to the computer, you should be decoding some stations.
Is the vertical bar in the lower left of the main screen reaching up into the green, but not going over the top into the red? If the audio levels are too low, or too high, this can affect decoding. Adjust the recording settings in whatever computer operating system you are using so the vertical bar is at about 30dB when there's no signal. You may have to tune to a quiet frequency away from the well known FT8 frequencies.
Also, make sure you have set FT8, from the 'mode' dropdown, on an FT8 frequency and not some other mode.