During the breeding season, they like to eat small quantities of supplements and egg food and they should also have access to a grit mixture.
In my aviary I feed my birds frozen mixed veggies- any variety is fair. Do serve the vegetables after boiling it in a pot (place the veggies in a pot, bring to a boil) and cool them by running cold water.
Some of the lovebirds owner place broccoli stems in a food processor or cabbage and carrot together- most of the lovebirds love it like anything.
The main prerequisites to make your bird hale and healthy is be patient and do provide your lovebirds a good variety of fresh veggies for at least three weeks or until your lovebird eats the treat.
Even you can use good cockatiel seed mixture with added raw sunflower seeds, 12 grain cereal mix, and wheat germ for the lovebirds feed. Even you can add about 2-3 teaspoons of oyster shells to each batch of seed mixture.