How do you interact with them? What is their experience/life like with you?
Respect and understanding is the most important part of bonding. Even though I know I only want to love and care for them, I understand my fids must learn that about me, no level of trust is inherent, but earned. I am an enormous predator and they are caged prey animals that cannot escape me when they feel threatened. It’s pretty terrifying to be housed in a confined space with such an animal (humans)!
My primary goal then is to introduce myself to them as I am (dedicated servant) and to give them every opportunity to learn that I am not something to fear. Ambient attention is a fantastic bonding tool, talk to them without staring at them (we have forward facing predator’s eyes), sing, read, laugh, be silly, describe what you’re doing and why; let them get used to your voice, sounds, and movements. I talk with my hands so they are able to witness them not as terrible grabbers, but simply as extensions of myself and always ambient attention is provided from a safe distance.
Cover a corner of their cage so they have a safe reprieve if they feel intimidated, I also make sure they have a toy big enough to hide behind. Millet is a great bridge for parrotlets. For it to work let them finish what’s in the cage and don’t replace it. You are the vessel that provides millet. I snip the spray into smaller pieces and start by approaching slowly, talking gently, and drop a piece into the bowl. Then back off, let them eat it, hopefully with you nearby but at a distance they feel safe with. Keep talking to them and showing them you are the bringer of good food! Eventually you will transition into holding the spray through the bars or door and offering millet from your hands.
Free flight in a safe room with plenty of perches and toys gives your parrotlet the opportunity to fly, play, and enjoy the space with you while learning to trust and push the boundaries of their proximity to you. Eventually (in my experience quickly) you are able to walk by them without them abandoning their perch to flee you. When they have the ability to escape you, they become emboldened to check you out! And so long as their door is open and the food and water is in their cage, they will always return home when they’re ready.
It takes time, patience, and the understanding that you are asking a very small, captive animal to go beyond instinct and to trust you. It is really a constant honor to be an exception to the instinct of a wild animal, and our fids are exactly that.