photoshop dropshadow doesn't work

Hello

I have added dropshadows to images before and had a similar issue I overcome somehow, but cannot think for the life of me how 😅 I have my image open, add canvas size round the edge of a white border, I have tried creating a new layer and doing it this way adding the dropshadow and also working on the original image, just one layer and both ways do not work. I use black as the colour of dropshadow on a white background, tried screen and multiply but no nothing works. The stroke works OK but cannot get the dropshadow to display. Is anyone able to tell me what I am doing wrong please?