When you jap a mantra, the energy that jap generates has to travel. It attracts the deva-tattva, draws on the energy
stored in the deity's pinda, and returns through your Sukshma Sharir and karan sharira. The discourses are direct
about what governs that whole medium of transmission: the Moon is its chief karaka. Which is also why
the Moon's placement in your own birth chart decides how a mantra attains Mantra Siddhi in the first place, and
when a siddhi already obtained may lapse.
That is the reasoning underneath everything that follows. Poornima is not merely an auspicious date on a calendar
— it is the point at which the medium your practice travels through is at its fullest. The corollary matters just
as much, and is stated plainly: mantras that confer ugrata belong to Krishna Paksha, Ashtami through
Chaturdashi. The full moon is for the other kind.