Re: San Marzano seedlings
Have you checked them for any pests? Sometimes spider mites will cause the damage you are describing. If you do see webbing beneath the leaves (the mites are usually too small to see clearly) give them a strong shower making sure to clean under the leaves. Shower them every day or so for a couple weeks to remove subsequent generations. Plenty of water and a fan blowing on the seedlings can help prevent another infestation.
Another possibility might be a nutrient deficiency. When you re-potted them did you use a fertilizer? Seedlings don’t need fertilizer until their first true leaves appear, so sometimes soil mixes and recipes advertised for germinating seeds don’t contain any. However it is easy to over-fertilize tiny seedlings so do check the soil bag/recipe beforehand.
For me it is better to start tomatoes and other seeds that get transplanted after the last possible frost in pots at least 3″ in size. This is because I typically need a lot of wiggle room during transplanting time; it ensures that even if things get busy (and for me they always do) you won’t have seedlings outgrowing their digs. I also use sifted composted horse manure in my soil mix to ensure there will be enough nutrients for their entire stay in the pots. Margaret has some great tomato planting instructions and tips here. And on the far right menu ‘Browse by Topic’ under ‘edibles’, click on ‘tomatoes’ to find a wealth of info on tomatoes covering from seed to plate.