What type of stem plant are you trying to grow? Plants are a funny thing when it comes to growing them in aquariums, especially since most of us are not
aquatic botanist.
I have found out after reading many a thread and my own experience that while some types of aquatic plants will grow and thrive in one aquarium it fails in another and vice versa. For me it has all been a matter of trial and error as to which plants grow well from me, and I have gone through several species figuring that out.
The only way in my opinion is to try and provide the best fertilizer and in my experience Thrive can't be beat. The right intensity of light and duration of light on and off. Your water chemistry plays and important part. When a plant fails for me I try something else,
Amazon Swords (You can get several different types of Amazon Swords, some get large others stay small) have been my best grower,
anubias has also been good for me, but for some reason ferns never survived.