Why can you see vapour trails only sometimes?

Its to do with the weather as planes all run on the same fuel, called kerosene which has carbon and hydrogen in it. When you burn it the fuel the hydrogen combines with oxygen in the air to form water, which then condenses to form long, thin clouds that you see. If the plane is flying through a particularly dry piece of air those water droplets will evaporate quite quickly, if its flying through air which is already quite damp then the water can't evaporate and the clouds last for ages.