I believe the BOG FP+ is still technically in testing phase, and you're supposed to have an email invite.
I talked to someone in tech support about it and he said that while the general public was accessing the website (since the link was floating around), if you only made your reservations that way (without the email) then you'd get denied entry.
Obviously lots of people are having success booking through the link (I did, I only called IT cause my trip dates changed and I couldn't figure out how to change them in the BOG website), but I've seen a couple of threads popping up around here with people getting turned away.
e.g.
http://www.disboards.com/showthread.php?t=3262235&highlight=bog+fp