If you deposit less than the minimum amount required (currently €10) to your Bitrefill card, the funds will not be added to your card balance.
In a situation where you have deposited less than €15 to your Bitrefill card, you simply need to perform another deposit so that the total of both your original underpaid deposit and your new deposit represents at least €15 in value.
Once you do so, both the original underpaid deposit and the new deposit will be shown in your card account balance.