Brand new masks will fog up quickly if you take them straight out of the box and go for a dive. A thin film of contaminants stick to the glass lens during the manufacturing and storage of each mask which make them more prone to fogging up. Here at Simply Scuba, for most masks we recommend pre-cleaners, such as SeaBuff, to prepare your mask when you first get home; this removes that film and improves the performance of your defog gel.
Before every dive you still need to apply a defog gel or spray to the inside of your mask to keep it crystal clear during the dive. There are other methods to reduce fogging, such as: filling the mask with sea water right up until you jump in so the glass is the same temperature as the water or wetting your face first to cool down your skin.