From the evidence we have in the leaves we can observe that the midribs are strong and lustrous green but the patches of yellow are on the outside of the leaves. Dracaena is a monocot, which tells us that the veins in the leaves run mainly lengthwise in the leaf, with little sideways translocation as would happen in a dicot which has a branching vein system. The veins on the outside margins of the leaf are thinner than the main veins.
The spotting could be related to water or nutrients, most probably the latter. If you are confident there have been no lapses in watering then we turn to fertilizer. We can see the leaf is deep green mostly, so we can rule out nitrogen deficit. However there are other micronutrients to take into account; if one or more of these is lacking then the big pipe down the middle of the leaf will ensure that it gets whatever is available, but the thinner tubes down the outside might suffer. Remember it is possible to have too little and too much of some nutrients. Sometimes it just takes one single magnesium ion missing to have an entire chlorophyll molecule fall apart.
University of Florida has a production note for Dracaena which mentions this aspect of micronutrients; it is mainly intended for commercial growers but we can learn a lot from it. Note the comment about fluoride and if you are using tap water with fluoride added you might want to source some rainwater or other neutral source. Otherwise make sure your plant is not root bound in its pot, and has adequate fresh compost to grow into. Pure peat type mixes are not very reliable since the micronutrients added might be exhausted. Try to find a potting mix with garden soil or compost added - the advantage there is that you will have a buffer of micronutrients the plant can draw on if necessary.
