ODD occurence with looping macros

OK,
I have a macro that allows the user to select a bunch of SSNOs, pop them into a form (which then populates with unique information), print, create a .pdf, then move onto the next SSNO in the list until it's finished.

It seems to work perfect, but we just ran a large batch of SSNO's (776). The macro only printed 773 of them, and only created .pdfs for 689 of them. There isn't any pattern (such as the first 84 or the last 84), but rather seems to randomly skip the process.

Any ideas?

Here's my code:

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Anyone have any idea why the macro would work only sometimes?