You don't tell us what you have connected to the RPi...USB, hats, etc. It sounds as though your load is enough to pull down the supplied voltage below the threshold.
You also do not tell us what sort of supply it is, other than that it came with your RPi. Maybe your seller shipped a supply with a smalller gauge wire or a lower rating or , even worse, a charger, as opposed to regulated supply. An official RPi supply will provide 5V/3A at the connector and the recommendation is provided on the basis of "average" loading. If it is not an "official" supply, all bets are off and you need to deal with the seller.