I would dig them up and divide them.

Can I do this after they flower and the leaves die back?

Yup, perfect time to do it is after the leaves yellow. Dig some of that compost down to where the bulbs are planted and they should bulk up again. I like to replant in a new spot since it seems some plants tire out the soil and will do well moving around every few years.

Thank you. I would love to do what I can to rejuvenate these as new ones are pricey. I was thinking of moving them anyway.

Over time, the growth of trees can make more shade on an area that had more sun a decade before. Maybe a factor?

Why yes---a giant pine has been steadily getting bigger these ten years. Time for dividing and moving.

Globemaster will not reproduce from seeds, as it is a sterile cultivar. Probably why they are so expensive. The bulbs split in half, and can be carefully separated and replanted in a sunnier spot. Given some time to adjust, they should bloom well for you again.