Answer: In my experience, houseplants go through an adaptation phase when you bring them home and the foliage may change somewhat in response to the home environment which is usually less ideal than the greenhouse where they were initially grown. The typically drier air and darker conditions will both have an effect on plants.
Spider plants will lose their variegation when they need more light than they are currently receiving. A very bright location with direct sun all morning is good, a south window is also fine, but a hot western window with sun all afternoon may scorch them unless filtered with a lace curtain. If you are using artifical lights, you may need to extend the time they are on.
Although you did mention that the sun porch was brighter, I am not sure why that would have made the problem worse unless it is only bright in say the very early morning or very late afternoon -- so the plant would still need more light. When you move plants, be sure to allow about six weeks for them to acclimate to the new location. Remember too that summer days are longer than winter days, so the light will be brightest in the summer.
I am not sure what kinds of plants your other ones are, but typically plants that need more light will lose some of the contrasting coloring or even revert to plain green, so I would expect this to be the cause there as well.
Underfeeding can cause weak growth or very slow growth. Overfeeding can also cause problems because the plants lack the light needed to process the nutrients and then develop overly lush and weak foliage as a result. Make sure you read and follow the instructions on the fertilizer and that it is formulated for foliage plants. Do not fertilize during the winter months when the plants naturally slow down and rest in response to the short days.
I hope this helps you get the variegation back.
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