I responded to your post over in the Houseplants forum, but then I followed you over here and posted it here as well...

Underwatering can be just as detrimental as overwatering. Underwatering and not watering are two different things. I have barely watered mine since bringing it back indoors for the winter, and it hasn't phased it one bit. But when you underwater a plant, then you run the risk of the minute traces of minerals in the water slowly building in the soil up over time, making it more difficult for any plant to efficiently absorb water.
Instead of just spraying your plant, I would follow LazyGardens suggestion and thoroughly water your plant and then let it dry out before watering it again. And it's not necessary to water the leaves either, given their somewhat waxy texture. They don't really absorb water water that way. This may be why you're having leaf problems.

I don't think that it's from too little light either. To give you an example, I had a leaf that had broken off of my main plant. To test its ability to survive (out of curiosity), I placed it in a drawer and gave it no water or sunlight. To see how long it would live for. It took it roughly eight months before finally showing signs of withering up and dying. For the majority of that time, it looked like a normal, healthy leaf.

I see fungus on the middle of the leaf. Cut it out. Spray with Hydrogen Peroxide in spray bottle. 2 cups water, 2 tablespoons of HP. Don't water plant any more. Soil looks wrong for Sanseveria plants. Needs more perlite or drainage. Keep in bright window but watch out for cold drafts. You may loose this one as well from crown rot. When in doubt on watering. Don't water.
Best of luck
Stush

Inappropriate potting mix.

Josh

I agree with everyone, but also have to ask if it is getting any direct sun on the leaves. I have only my tiny phone screen to see the photos, but from what I can see, there could be some sun damage. You'll have to be the judge.

Sun damage takes ages to appear in Sans, but those spots turn pale, then Yellowish, then get soft, and then dry out and become brown. The change happens only on the parts of the leaves exposed to too much direct sun. The rest of the plant remains healthy. The problem is exacerbated by salt build-up in the soil, too much fertilizer, or too much or to little water. I can't see well enough to judge, but it's another possibility you should eliminate.