This is usually due to one (or both) of the following reasons:
1. There were multiple winners, so the prize must be split among them. For example, in a £5,000 1X2 Pick 5 pool, if you are the only winner you would win £5,000. If there are 20 winners of the pool you would win £250.
You can see how many winning units there were in any recently settled pool by clicking on the results tab and looking at the "Prize Breakdown" for each pool listed.
2. You chose a stake per line on your ticket that is lower than the default unit stake in that pool. For example, in a 1X2 Pick 5 pool, a £2 stake per line plays for £5,000, a £0.50 stake per line plays for £1,250 and a £0.20 stake per line plays for £500.
You can confirm your stake per line and amount you were playing for by looking at your tickets in the "My Tickets" section.