It seems most (or all?) Blyxas need more time to establish.
I've just realised that my Blyxa is Novoguineensis, not Japonica.
So can't chime in much, but I've noticed many tanks here in Thailand
grow it (Japonica) fine, very fine indeed, at temp around or higher than 28C.
Heck, Blyxa in natural habitat here (don't know what species is
native to here though) grows in very high temp, under blazing sun in
rice paddies and streams.