It is possible that you are running into file locking where while the image is being loaded, it prevents another thread from trying to use the same image. How are you setting the image source?
This is what I do for my application...
try
{
using (FileStream fs = File.Open(imagePath, FileMode.Open, FileAccess.Read, FileShare.Read))
{
symbol.SetSourceAsync(fs);
}
}
...