It's difficult to say what causes that because an ad being blocked/not blocked depends on a lot of factors (e.g. Adblock Plus version, installed filter lists, filter list versions, custom filters, YouTube itself, geolocation, etc.) so the next time you encounter those ads again, can you please send us an issue report using the "Report issue" button in the toolbar icon popup?
That way filter authors can have a look at all the information about the problem, try to reproduce it and make the necessary changes to the filter lists so that those ads will be blocked again.
Alternatively, you can report any ads that aren't blocked to the filter authors at EasyList directly via their GitHub page
or their forum
You can also check out our Help Center website
which is where we're publishing further information about and troubleshooting tips for Adblock Plus.