Noel,
If the dogs can feed with light pressure, then they do work and should not need replacement. I suspect from what you have said that when they are in the up position can you push them down below the bed with your finger. You should not be able to move dogs by finger pressure. Follow the path of all that connects to the dogs to find what is moving.
It may be in the feeddog lowering knob/ lever. If that does not them lock securely in the up position they won't work
under pressure.
Cathy