The missing cards are most likely cards that it can't find adequate multi-choice prompts for (this is a multi-choice test, right?) - can't find enough other cards that don't overlap with them. I'd suggest that you do a "self-graded" test with a "card filter" enabled to limit the test to cards that were never reviewed (or not reviewed within the last, say, month) - that should let you test on the missing cards.
Alternatively, they might be cards that are incomplete - missing definitions or whatever; you can search for cards like that via the Organize Cards screen. (search button / search for 'incomplete')
