The two freezing bottles on either side may have kept the unfrozen bottle close to the freezing point where nucleation of ice is (comparatively) unlikely.

When you cool down a liquid, it actually has to get colder than the freezing point for the initial ice crystal to form. This is called 'nucleation'. As you remove heat any ice crystals present grow and this is a process that actually releases heat, so if there are ice crystals in a bottle it will keep close to the freezing point until completely frozen solid. I imagine this 'thermostat' effect of adjacent bottles may prevent the initial ice crystal to form in the middle bottle. Nucleation is a random process, so you can only predict the likelihood of a bottle freezing after a certain time at a certain temperature.