Probably a question of time and patience. My advice would be to be in its' company but don't push space boundaries. E.g. be in the same room doing your own thing, maybe sometimes speak to it, look at it and slow blink and just carry on with your own life.
Offer food and treats in your company (same deal, give food but stick around in the room, but give it space). Sit a lil bit closer to it over time to build trust but avoid initiating touch. Try to play (like string on a stick or something similarly simple), some cats value playtime over food. One of my cats values just being stared at over both food and play, they each have their own preferences - once you figure it out it'll be a lot easier.
Eventually it'll learn to trust you and associate your presence with food and play.
Just be aware that it may take weeks or months to build a bond, although if you aren't seeing any improvement at all (like cat is tense when you're around, even from a distance, reluctant to eat when you're near etc) in 2 or so weeks time then the method isn't working and you gotta try something else.
Good luck, and thank your for caring for it!