this has been a problem of substrate and pot. Substrate too compact, little loose and pots without aeration hole, the water remained stagnant and the roots began to rot. I advise you to buy black and blonde peat substrate and mix it with perlite.
also buy a pot and make 4 holes on the sides so that the substrate remains aerated and if you want you can add a small ball of clay as drainage.
check the ph at 6.0 ok.
Greetings and merry christmas.