They may be a few reasons for this. Check to see if it's any of the following:
1) You have reached your deposit limit. Check by going to ‘My Account’, click ‘Safer Gambling’ and select ‘Deposit Limits’. Here you can check what deposit limit you’ve selected and whether you have already reached it.
2) You have selected the deposit limit to be applied to all linked accounts on our sister websites. You can check this by going to ‘My Account’, click ‘Safer Gambling’ and select ‘Deposit Limits’. Here you can see what deposit limit you’ve selected and whether you have already reached it.
3) You tried making a deposit that was unsuccessful and you are trying to deposit again. If the deposit attempt is in a ‘pending’ status, this will affect your deposit limit.