If you deposit less than the minimum amount required (currently €30) to your Bitrefill card, you will not receive the funds in your card account balance.
In a situation where you have deposited less than €30 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 represent at least €30 in value.
Once you do so, both the original underpaid deposit and new deposit will now be shown in your card account balance.