Dear Helene, sorry to say but this is a very common experience many breeders have to endure. And mostly there is no explanation for it. There have been broods with 6o nagyi, all female. Or broods with 70 linkei, all male and only two or three female.
All or nearly all environmental factors could play a decisive role in finally fixing the sex of a young fish developing from a larvae to ab age of about two or three months. We know that for instance pH, temperature and the content of humine substances coud affect the sex ratio of the very young fish. But we cannot say definitely which is responsible in a certain case except there is some really borderline value indicating that it is probably the causer.
Did you control temperature? Was it especially high or low? Have you been conscious about the amount of humic substances? P. harveyi is especially bound to the typical blackwater conditions, more than nagyi or linkei or spec. Sentang, not to speak of paludicola. It will spawn at several pH-stages (all well below 7.0) but all of these are compatible with more or less shares of humine acids. We also know that typical blackwater-fish (just as harveyi) do often spawn in clearwater, too, and we do not know to which effect.
At any rate, I use that “Eichenextrakt” too, but I am not sure that it contains the same qualities in every charge. To my knowledge it does contain little “Eiche” (oak) but much phosphoral acid. It must be manufactured cheaply. Nevertheless I am rather content with it in most cases.
But I am not sute that this is the cause of your one-sex-experience. Without heavy experimenting we will not solve the riddle. Sorry, but youz are memmber of a big community of experinenced breeders woho all share your problems and – in the end – don’t know what the reason is…