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 will win £5,000. But if there are 20 winners, you will win £250.
You can see how many winning units there are in any recently settled pool by clicking on the "Results" tab, navigating to the relevant pool under "Completed" and looking at the "Prize Breakdown" for that pool.
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 ticket in the "My Tickets" section.