Some species of fids like certain types of pellets more then others. Budgies & Teils seem more drawn to Tops brand which are an excellent pellet in my opinion. I got them in April (I think) & I'm still trying to get my Zon to eat them. Sizes of pellets are important too.
His next endeavour will probably be Harrison's. Higher fats content, he'll be more likely to try because of the peanuts & sunflower in them. I so wish he liked the Tops! He's had a basically fresh & sprouted diet since april. Try, try & try again. Persistance & patience us all we can do. Remember though variety is the spice of life. More ingredients in a fresh dish is more vitamins & minerals in their nutritional pool so to speak.