By the way...please do not walk up to the check-in, with "I tried to check-in online, it didn't work. Can you tell me why!?"
No, the agents can't, most of the time! They are not IT.
If they see the selectee-marking on the BP, it's clear; but so can you!
Maybe the passport/visa-data was not filled in correctly online; it is rarely an actual ticket issue.
Not being able to check-in online should not be the big drama, that some pax claim it is. If you check-in luggage, you'd have to see an agent anyways.