Hi,
It's probably because you have more than 500 products/variants and that you didn't change the "value/limit" fields of your mass action. These fields limit the quantity of the products/variants to be updated (because updating too many of them at the same time would make the mass action crash in the middle because of the restrictions in place on most hosting plans) to the first 500 elements. Change the value of these fields in order to update the rest of the elements.
For example, if you have 2000 products/variants, set a value to 500 and a limit at 1500 in order to start after the 500 you already processed and do the next 1500 elements.