Fertility in the Bengal and Savannah cats are often talked about in breeding. Bengals and Savannahs can be fertile as early as 5 month old, but can also take up to 2 to 3 years to become fertile, although its more normal for domestic female domestics to be fertile at 12 to 18 months, and male domestics to be fertile at 12 to 24 months. Once fertile this fertility can be interrupted due to stress or illness. Once a males fertility is interrupted it can then take another three months before fertility gets back to normal. If the stressful situation hasn't changed then the fertility may be limited in that environment. I once waited 3 years for two sisters to become fertile and then neutered one of the sisters, the other sister was to be neutered once she stopped calling but - 2 weeks later she was found to be pregnant!
Stress triggers can be many things such as traveling, change of environment, winter months with the change of temperature and change of light, noisy parties, noisy children, new dog moving in next door, new children next door, football being played against the cat pen/chalet (no you say, but you would be surprised!) and the list goes on - its up to the individual to assess the situation and work out what is upsetting the cat.
Early generation Savannahs (or Bengals) usually take longer to become ready to produce a litter than your normal domestic cats. F1, and F2 females inparticular can take longer due to their stress levels, and the male F1, F2, F3 and 50% F4 males are infertile. The same applies to Asian Leopard Cats and African Servals - its not necessarily that they aren't fertile but that they aren't ready to produce a litter.
Be Patient! your cats will produce you some wonderful kittens if you show them understanding and loyalty.