With coco perlite, you will always want to ph your water. Also, coco is notorious for causing mag deficiencies, add a teaspoon or 2 of unscented Epsom salt to your water every other week.. With that sorted, your growth should improve. Cannabis is a heavy feeder. Before long, I think you will need to add more food. I like to top dress with a cup or 2 of worm castings and some all purpose dry organic fertilizer. It's slow release, so it won't affect the plant right away, but in 2 weeks you will notice lots of healthy growth. It also helps buffer the roots against massive ph swings.