The signal then stays about at 0 mv, sometimes a peak is seen but not the baseline.
If the baseline has been zeroed when the signal was high it can then drop below the range of the output and appear to go below zero on the scale if the background scattering becomes lower as the run progresses.
Try this first - when you first power ON the detector and select the RUN mode you will see the display showing a number that reflects the background scattering of the optical bench. Before flowing any mobile phase, please report the number. Press the A/Z button. The number should come to rest at 0010.
Your gradient may be producing a high background signal that then becomes lower as the composition changes over the course of the run. If this change is very dramatic its possible the solvents are contaminated and require replacement or filtering.