Sometimes they scoot back up into the abdomen
To be honest, mine did that just from reading this thread!
We found this quite tricky last year with our Manx Loaghtans, since they hadn't descended by 5 days old, but by day 6 the lambs had learned how to run! Also, once descended, the testes were so small, it was really difficult to be sure of catching them with the ring.
In the end, we tied a loop of string in a single half hitch knot around where the band would eventually go. That was enough to make sure the testes stayed in the scrotum while the ring was applied. We aimed to place the ring above the string, before cutting the ends of the string, which meant the knot then fell undone. Not an easy job, but at least doing it like this meant we didn't have to rush, and reduced the risk of trapping balls in the rubber band.
The only other lesson was to be really sure how many of each sex you've got. We evidently mis-counted, because when we came to give them their first Heptavac a month later, I turned one over and exclaimed "b*****ks!". "They certainly are", replied Mrs Womble