Daniel g posted well, they think your some type of predator and are afraid of you , they don’t know you don’t want you to eat them. There’s no language you can speak other than time, patience and kindness.
There’s no training or taming a turtle, they will become used to a routine, especially cleaning time and meals. They eventually figure out when foods due and will eventually start coming out of their shell instead of being terror stricken in that strange alien nasty place you have them... meaning it’s not the environment they grew up in with everything’s familiar and they know, all these strange colors, punks, blues etc, white like snow but no cold., really strange and possibly dangerous.
There are no tv’s no video games, radio and lights in ponds or streams, they don’t know what these terrifying new things are they don't smell familiar, sound familiar or look familiar but will acclimate to them months from now not days. They don’t have even a cat or dog intelligence, they are more primal, eat or be eaten etc.
Put yourself in their place, you go to sleep, this giant grabs you and twirls you around and around shaking you, disorientating you, slams you in a stuffy box then whisks you away to an Alien place where you recognize nothing, no food, no resting places or hiding places, and then a monster giant keeps poking at you shoving some type of smelly thing in your face (food). The monster gets upset, you can smell it on them but you don’t know why, hopefully the monster won’t pull you from the shell then eat you.