Why is it that I see my Product SKU on Sales Orders but not on Purchase Orders when I issue them?

You can make the SKUs visible on your Purchase Order by editing your Brand settings.

Simply navigate to the Settings tab, followed by the 'Branding & Documents' tab. Choose the Brand you wish to edit and click on 'Document Settings'. Scroll down to the 'Document-specific settings' > Columns, and tick the 'SKU code' checkbox.