I think i know, same thing happened to mine had to take 2 off and restart those 2 seeds, glad it happened because turns out the cause was the soil being too light and i didn't realise this when i bought it, so i used a hotter soil and the 2 thst were replaced and now currently twice the size of all the others who stayed in the light soil.
I recomend finding a really old falled and rotten tree spot and digging out that soil as it will have a lot of microbes that are now feasting on the tree's leftovers and also will have lots of nutrients. Mix that soil in if you can or try to dissolve it with water and water them with that watered down mud mix. Will have lots of natural nutrients and may make them start growing again, but since they are auto's i would just dig the one that stopped growing out of the pot and mix the good soil with that soil and then popping another seed there or leaving it for when the plant on the side expands looking for nutrients to invest more energy into spreading a stronger root system wich would sort of go sideways but since it must expand it eventually makes her grow even bigger.