While I concur the manual is very useful place to start, a few other questions come to mind.

Are you testing this thing in the darkroom, illuminating the probe by the light of the enlarger?

Some analysers can 'saturate' and then stop working for a while when exposed to regular room lighting.

The other explanation is that the photomultiplier tube, which I am guessing your unit may contain, may have cooked off on power up.

Do you have experience with other analysers, to confirm how to expect how the thing is to work, or is this your first analyser?