Data not low enough to buy add-ons

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I recently wanted to top up data for the phone because the add-on I purchased beforehand was almost out, and I didn’t want to have to pay extra data charges in case I exceeded the limit.

But for some reason, my request to do so kept getting denied. So I called the helpline to ask what the problem was and the response was that I had to wait until I was very close to finishing the data before I could top it up.

My question is: Why is there no option for people like us who simply want to buy add-ons even when there is still some data left in our monthly allowance?

Puzzled