Cards can be declined for a number of reasons and it is not always because of low funds.
Due to the new 3D secure authentication methods, it is common for banks to block transactions - unless it is authorized either in your banking app or through text.
Some declines will not show on your bank account, these are known as Soft Declines. Hard Declines will always show in your bank account.
Banks are extremely aware of customer's spending and habits, some banks may flag a transaction if they feel it is suspicious or out of character.
E.g If you mainly only use your card for withdrawing cash or spending at physical shops and then quite a large transaction comes from a website, this can be flagged.
Some banks may also limit larger transactions if you primarily make small transactions most of the time.
If your card gets declined, we can provide basic information regarding the decline however in most cases you may need to contact your bank.