It's not well documented by MS (or at all, maybe), but copy-only fulls will not allow log backups to be taken if they are the only full since the DB was switched to the full recovery model.
Someone asked Paul Randal about that at http://www.sqlskills.com/blogs/paul/misconceptions-around-the-log-and-log-backups-how-to-convince-yourself/, and his explanation was this:
It’s because any copy-only backup does not affect *anything* to do with the database, so the copy-only full backup doesn’t set the LSN field in the database metadata that is the pre-requisite for moving from pseudo-simple recovery model to full recovery model.
Cheers!