Most likely overwater. Two suggestion:
Fill the cup with soil. Happy roots = happy plants.
Seedlings only need a coupe ml of water per day. Overwatering causes the roots on seedlings to struggle. It is called root rot or damping off.
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This is what I have left. Are the stalks strong enough to support themselves? Should I add some soil to the cups Any advice appreciated Should I transplant to organic soil?
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First one is typically “damping off” we call it.
Comes from over watering and it is the #1 mistake new growers make so do not feel bad my freind you are not alone 
Do the ones you have left have lots of good size holes in the bottom of the cups for drainage?
If not this is a problem as the water you give them has no where to go and tiny seedling do not use much water at all. Like 10lm every few days or so, its really miniscule what they need at first… just remember that in the future, but in my opinion the ones left dont need to set in soaking soil if my suspension is correct with over watering.
the ones you still have could be transplanted into there bigger pot now if you wanted to.
Just get your next pot ready and cut the bottom of the cups off and place cup and all into fresh big pot of dry soil. It will look something like this but a cup rather then a bag.
This should allow for no transplant shock and the dry soil and gravity will help pull some moisture out of the cup.
In a few days when you would possibly want to water them, you would water around the cup into the big pot not down into the cup.
This helps not to drown the plant and encourages the roots to reach out for the water…
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Thankyou for the detailed answer will do as u suggested When I transplant should I use organic soil?
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Organic or a good cannabis soil. Cannabis is sensitive to pH and needs a soil specifically pH buffered for cannabis. Fox Farm products, Roots Organics, and Black Magic are examples of good cannabis soils.
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You’re very welcome my freind.
We all love growing cannabis even if it isn’t ours we want to see everyone be successful 
Unfortunately im not any help with “organics”.
When i do use soil, I use Foxfarm ocean forest with ectra perlite added and after 4-6 weeks I begin feeding them mineral based nutrients “synthetic”.
I’m more of a salts guy myself, its much easier for me personally but lots sware by it but its mostly hype to sale products imho…
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Here we are on day 2 of transplant with organic worm casings Soil not packed it’s loosePreformatted text
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