I'm sure you know this already - but are you using your online account
password, which is different from your Oyster card security question/
answer?

Also, what's happening when you try and log-in - do you get an error
message. I've just remembered that I've had problems like this before a
few times, with a misleading error message appearing (I can't remember
what it said), but it's just been a temporary issue. I just presumed
that some unadvertised maintenance work was going on, and when I tried
later or the next day it worked.