A couple of things. Did you let out most or all of the full length of the rope attached to your mudweight? Beginners often keep the rope too short - it is best for the pull on the mudweight to be as horizontal as possible (within reason, of course!). Also, when lowering the mudweight once you get it at or close to water level let it go all at once rather than lowering gently. That gives it a chance to bed into the mud.
Lastly, was this on Oulton Broad by any chance? It's the only place we've ever dragged, I suspect the bottom isn't as muddy as most other spots, at least in places!