Don't know why one of the seedlings is doing like that , germinated after the others and stopped immediately.. after two days restarted to grow, but veeery veery slowly, i'm thinking to replace the seed but i'm not sure at all...
The problem might be the low temperatures in your grow space! 21 degrees Celsius is quite low for a seedling, they usually need 25-28 degrees with some varieties even 22-26.
You should always aim for a higher temperature during that early stage. About 23-24 degrees would help your seedling grow better.
Also try giving her about 1 liter of water with 1/5 of the recommended fertilizer dosage.
Hope this helps! Good luck and happy growing :)