Nandrolone will suppress you at 2 levels due to the progestin base as well as aromatization to estrogen.
1. Reducing LH/FSH release from the pituitary by binding to androgen receptors and suppressing GnRH release from the hypothalamus. (GnRH is what our brain sends to the pituitary to start making LH/FSH).
2. Binding to estrogen and progestin receptors - downregulating GnRH receptors on the pituitary.
So you get a double whammy, specifically a more significant one due to the pituitary receptor downregulation via progestin.
This *could* be why some have lingering HPTA suppression, even after a proper PCT when using progestin based steroids like Deca/Tren. The downregulation of receptors on the pituitary could make you less responsive GnRH stimulation (which we indirectly achieve by using a SERM), leaving you producing lower amounts of LH to increase testosterone.