Having issues with slope analysis. I click raster, analysis and then slope.

I enter my input and output and run the tool. The output I get doesn't appear to have been calculated correctly. Instead of seeing a continuos gradual change in slope through various colors, I just see a scattering of small squares.

It looks like only the max and min values are being displayed an nothing else (not sure).

Do I need to use the terrain analysis plugin for the tool to work? Or is there something in my workflow that I need to change/add? I am using version 3.0.3