I heard from one of my favourite ITks (someone I know at Arsenal who delights in showing me e-mail trails between him and Gazidis to make him look important) today that the reason there's an issue with the supposed release clause is because of the concept of the Champions League. He didn't back any of the following up with evidence, but it's an interesting theory at least.
This clause exists but Liverpool don't actually think we're in the Champions League, because we haven't negotiated the qualifier yet. Arsenal think they are, and thus the transfer has been held up while the lawyers get involved.
Therefore, Arsenal are refusing to pay any more than £40m on the basis that a) they think they've offered enough and b) if we win our qualifier, then we will definitely be in the CL, and then we'll have 4 days to complete the deal anyway.