Think about it. 48" of surface area being pushed VS 8 tiny little points. The problem with your bucket may be just that it has a really wide surface area of the leading edge that the wet manure and grass grabs on. Put teeth on that large bucket and you will be able to get into the pile easier.
Many times when I cannot get my large bucket to get into a pile of material, I switch to the small bucket with teeth and have much better results. You can probably pick up some sort of universal bolt-on tooth bar that would be easy to remove if needed.
Just something to think about.