Hi @NigelBerjak ,
Is that layer public? Or simply on ArcGIS Online within your organization? If you share its URL I can be specific, but here is some general stuff. Depending on they type of layer, enabling the pop-ups on that layer should reveal their pixel values if they are a imagery layer from ArcGIS Image for ArcGIS Online or an image service. If the layer is a tile cache or map image, then the pixel values will not be visible.
Here is a map image layer (map service) so this one will not have access to the pixel values.
A quick reminder is looking for the Pop-ups button amongst the properties for the layer. As you can see here the pop-ups are not available.
The layer I used was from ArcGIS Image which makes all the cell/pixel values available. You can see the icon for the layer is a little different.
So if you do not own the layer, but want to see the pixel values, you have to enable the pop-ups here.
If they are available based on the layer you have selected. So not all layers have same qualities or abilities, but take a look at ArcGIS Image layers if you want to see those values. In this blog, you can read more about some of the advantages.