I used to rehab turtles and sometimes when they are diseased or injured they cant catch the healthy fast fish, natures plan you know, weeding out the weak. To make it easier you can remove part of the pectoral fin of the fish, it will be easier to catch. Or simply try a larger easier to catch prey, some small feeded godfish are just too fast, shop around.
There are many store bought dry turtle foods that offer a balanced diet, avoid dog food as the excess protien will cause shell problems. Also avoid lettuce as a meal, it has no value except as fiber, in moderation it helps digest but wont sustain the animal.
Post a pic I can tell you what type it is if you dont know already