Imma keep it real with you growmie.
The strain is called dwarf but its not expected to be this little by this point, i know the feel of being stuck with the midget and watching it not grow everyday without knowing the reason.
My best guess would be that the soil is causing this problem, and im affraid this ain't no guess.
Go to a grow shop's website and order some top quality soil, imagine you are going to the dealer and now is the time to decide, do you want high or low quality, and you only get to pick once cause he will deliver the ounces of the quality you picked at first and you won't be able to change it after, you can use nutrients if the soil is light, but if its low quality overall then you can't fix.
I suggest you get a 50l bag of a "complete mix" not the heavy one nor the light one, go with the "Complete mix", there is almost always a "Complete" option.
place the seed straight in the soil, and then watch her grow fast, fast like sanic.
If you do just this and you grow outdoors, you only need to ph the water after this and you can still reach a medium harvest, if you keep on that soil, you may have issues even getting 20g in 3/4 months. If you are indoors you have to make sure the other aspects like temperature being synchronized with humidity, and relatively stable, so you dont want temps going from 40ΒΊC to 17ΒΊ in a day, but if its 25ΒΊday 20ΒΊnight the its ok, even if its 35ΒΊ during the day and 30ΒΊ at night you can still grow. The higher the temperature the higher humidity she can take, altho you can't go to huge levels, just follow this chart to try and keep it at the correct transpiration rate. (link bellow is a picture, i suggest you download it)
growdiaries.com/diaries/55942-fastbuds-purple-lemonade-grow-journal-by-pmal027/week/294074
If indoors don't keep the light too close, don't take the plant outside during the day for sun light and then back indoors for artificial light at night its doesn't work like that.
And... be patient, idk.
If you need anything just dm me.
Best of luck and Stay lit growmie.