The success of any endowment policy is linked to the stock market, and will vary from year to year. There are risks involved in any investment that is linked to fluctuations in the financial markets in this way.
Many endowment policies that were taken out when interest rates were high (for example, between 10% and 12%) were issued on the assumption that it was reasonable to expect them to earn between 7% and 9% each year. However, when interest rates fall (for example to 5% or 6%), there is an increased risk the policy will not earn as much as was predicted.
Many borrowers who took out endowment policies in the 1980s or 1990s to repay their interest-only mortgage are finding that their endowment will not cover the full amount that they borrowed.
Speak to your lender as soon as possible if you think this applies to you. You might find that you are left with a large shortfall at the end of your mortgage term.