Adrian
As for your question...

"Just for my curiosity, is there a trick I missed?"

I also used the Ross Round suppers for the first time this year and noted the
same condition you were talking about....   It took the longest before my
bees would start working my Ross supers.....  I would also mention that all
four of my supers were new....  Don't know, it could be the plastic frames
the bees have a hard time accepting...  Once work started it didn't take long
for them to fill up and cap off.... Hope I do as well next year....

Paul