I am having trouble booking Singapore stopover via the Singapore Airlines website, any suggestions on what to do?

I am trying to book (or at least waitlist) a return business class trip for 4 to Europe over December/January via Singapore Airlines and when I try to put in a stopover in Singapore on the return journey, the site has a little meltdown and doesn’t seem to like what I am doing.

Is there a trick to this that I don’t know about?

No trick I know of either! This is a relatively new function on the SIA website (stopovers, that is) and I have had issues as well. The KrisFlyer call centre is pretty good about booking stopovers over the phone and respecting the online booking discount if you let them know you couldn’t book online in my experience. Try calling instead and see how you go.