Hey DJ,
Your sliders will only eat while in water. Sometimes, they'll pick things off of the shoreline but they will always drag it back into the water in order to consume it.
You are on the right track with the food items that you are offering. Go to any pet store and purchase a few bunches of Anacharis(Elodea). This is an aquatic plant that should thrive and grow in an outdoor pond. To my aquatic turtles, this stuff is like crack! They love it. That way, you'll have a constant supply of food in the water without having to worry about fouling.
For fussy feeders, you can try boiled chicken breast or earthworms. These can often spark a feeding response.
The biggest issue for any wild caught adult turtle is stress! A turtle will literally starve itself to death if it is too stressed. Granted, this usually requires absolutely horrible conditions. There are varying degrees. It may take up to a month for turtles kept in the best of conditions to acclimate to their new surroundings. In the meantime, you need to look at ALL of your husbandry practices in depth! Any of these things can affect appetite:
- Do they have access to direct sunlight in which to bask? If so, how many hours?
- How many gallons of water are you providing?
- What is the temperature of the water first thing in the morning, at noon, and at dusk?
- What are the air/ground temperature highs and lows in the pen throughout the day?
- Are you handling them often?
- Is the water filtered? Is there a current?
- Are they actually basking?
- Do you have an outdoor dog that spends a lot of time near their pen?
...just to name a few.
Sliders will feed throughout the day. As adults, they do a lot of "grazing". They will bask(heat up, digest their food), then drop in the water and eat, then bask, then eat, then bask...you get the idea. Temperatures and lighting must be correct in order for them to properly bask and digest their food, which affects their appetite. Throw in mating and going to the bathroom...the life of a slider in a nutshell.
A picture of your setup, with the turtles in it, would help.
Symptoms of starvation vary and can take a VERY long time to show up. Unseen symptoms are more dangerous, like damage to internal organs. The absolute BEST thing for you to do would be to monitor their weights weekly with a gram scale(postal scale, chef scale, etc.). A 10% drop in weight over a month combined with anorexia may warrant a trip to the Vet. for fluid therapy or force feeding.
I hope this helps.