There could be a lot of things wrong. I generally keep maps isolated for longer than the standard health quarrantine period so they get used to eating and seeing people around before they are introduced to other turtles. And among maps some species are more shy than others. Does the turtle have access to an under water hide? Ids there a basking spot large enough for it AND the slider to bask at the same time?
Is the turtle being kept ina glass tank? Is it at eye level or is it low near the ground. Shy turtles which are startled easily some times never relax if they can see people all the time or conversely if they can never see them and are then surprised by their arrival.
How old/large is the turtle. If its a hatchling it may need smaller, slower foods. If it si a large female it may prefer larger foods, or crustaceans or mollusks.
What kind of map turtle is it? Several species will prefer crayfish, crabs, snails, clams others will be intereste in crickets superworms, beetles, grasshoppers. But by all means try several foods: tuna, pre killed minnows, live or cooked shrimp, several kinds of bugs, earthworms, snails, almost any kind of seafood.
I think the addition of some water plants and caves will help to make the animal feel more secure no matter what the situation but it may end up being beneficial to remove it from the same cage as the slider.
Then of course there are possible health issue. Does the turtle exhibit any other irregular behavior? (swimming lopsides, puffy eyes, scratching eyes often, floating listlessly, bubbly nose?)
Perhaps a vet check is in order.
It may help to raise the temp of the air and maybe also the water bya few degrees, as sometimes this stimulates feeding in picky eaters.
Try a few things and perhaps try also to do these things while the slider is out of the tank. See if it eats then. Also consider offering the food and leaving the room; a shy turtle may prefer to eat when you are not around. If nothing works in a week more, i'd find a good REPTILE vet.
-best of luck