Are you trying to wear her in cradle position, or upright in tummy to tummy?
Is your ring sling heavily padded on the rails, and is the tail sewn shut or is it open?
If it's an open tail, and unpadded, you should be able to adjust it to support her no problem.
For tummy to tummy, just pop her in and adjust either the bottom, or the top, or the middle independently. Her head will likely be facing sort of looking up at you sideways, and you should be able to tighten the middle to pull her back in toward you so that she's straight, not curled up. Once she's in, if you hug her and she moves closer to you, your sling is too loose. She should be as tight as a hug in there.
For cradle carry, start in tummy-to-tummy, and then move her feet/bum down and to one side. She will wind up lying diagonally across the fabric, and you should be able to tighten the middle of the fabric, again, to support her back and bring her neck into a straight line.