Vegetable seedling transplant problem

As a beginner, I decided to experiment. Until now I have found that starting seeds in a glass cup filled with aerated water (hydroponically one might say) is an optimal choice. At least it seems to work for me.

I then carefully transfer germinated seeds into holed polystyrene cups filled with constantly moist sand. The seeds are buried into the sand and then occurs new vigorous growth. Here I must note that the cups are placed in my room (near a SW facing window and day temperatures around 20 deg. celsius)

My goal was to transfer the seedlings into an already prepared raised bed, as soon as the cotyledons were well established.

Luckily, I did a test transplanting first. Following all the instructions I could find on the internet, I transplanted some spinach, lettuce and rocket seedlings into pots filled with compost and sand, only to see the little plants collapsing (wilting to the ground) in just a few hours. Note: the pots were left beside the sand-filled cups, that is in the very same environmental conditions.

• The compost & sand mix was kept wet but not soggy
• The plants were handled from the leaves
• The root damage was minimal as I immersed the cups into water and then simply collected the seedlings

Any ideas what causes this?

Could it be because there is low light and the plants have become "leggy"?

Or is it that such transplant is doomed to fail?

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[size=medium]Hello Rick and welcome to Gardeners Corner.
That's an interesting way of doing your seeds.
I think the problem you have encounterd is because you have transplanted at the cotylodon stage. Plants don't really have effective roots until they are into their first set of true leaves. I think if you leave your plants a little longer before transplanting they will do better. There's really no rush.
See what others think.
Good luck with the experiment.
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I keep my seedling compost moist by using a dilution of camomile tea which helps prevent damping off. I have had fewer failures so far this year.