What to do what to do....

So I have two plants growing right now. One doing amazing since day one and one been struggling since day one. The older one has been giving me a hard time since day one and still isnt looking great. the younger plant has since caught up with the older one and grown much bigger than the older one. she is in a smaller pot so im probably gonna switch the pots and put the older in the smaller and the younger in the bigger. At first I thought it was an autoflower because I tried topping her and it didnt work but I just decided to go with one big cola on her since I topped the older plant and she has been so rough. I treat them about the same the only think I could imagine that im fuckinng up on the large one is too much water. I have recently switched my soil and everything so its really getting down to be the only thing left to change. do you guys have any input or an opinion about anything ive got going on? Im trying to grasp as much info on growing as possible so all your help i greatly appreciated guys!

btw top and bottom pics are of the same 2 plants.

they really don't look to bad to me but I am a newbie. I have three seedlings I just put in 3 gallon pots and they all where popped the same time one is really slow and the other two are taking off it just might be the strain, are they the same?. I think I put mine in to big of pot but my other two are doing fine. Sorry I couldn't be of more help this plant can drive us crazy, they are pretty resilient keep us updated. I am sure there will be someone with way more knowledge than me to help you figure this out. Just wanted to let you know your not alone good luck keep us informed.