It is frustrating when this happens and unfortunately it can sometimes be hard to find a cause. It could be that the spawn was not fertilised. This often results in the spawn turning cloudy in colour and sinking to the bottom of the pond. Or it could be down to the environment. The two things that all spawn desperately needs to develop properly are light and warmth. You need to let the sunshine in to your pond for tadpoles to thrive.
Occasionally, the top layer of spawn can be caught in a frost and killed, although often the spawn under the water can still survive. It is also worth remembering that spawn is very vulnerable to predators - your spawn could have become a tasty meal for fish, newts, grass snakes and birds.
Try not to be too disheartened. Even if you don't have a successful batch of spawn this year, frogs, toads and newts will still be using your garden and pond for food and shelter. A pond is a fantastic way for you to help wildlife in Sussex. So keep the area amphibian-friendly and you will no doubt have lots of successful spawning seasons in the future.