I've seen a couple times when guys had pH that was way off before they started, self buffering newts couldn't handle the job. All they had to do was PH down there water before putting in their nutrients, that could be your problem. You can also have a high degree of total dissolved solids in your background water and that plant may just have too many salts down in the roots now.
Sent from my SM-G975U using Tapatalk