It sticks when it's at just the right moisture level. Super dry and it falls out, super wet and it falls out. I have a 24 inch bucket and I've tried everything ever posts on the internet. Nothing works. Lubricating the bucket only lasts a few cycles, then its sticking again. You would almost have to lubricate it after you dump it every time. I tried special paint that was guaranteed to stop anything from sticking to it, which worked until it wore off after half a dozen loads that cost $100 a gallon. I tried only filling my bucket half way and digging shallower cuts, but that just slowed down how fast I could dig, and still stuck to the bucket.
Run a hose in the ditch and keep it super wet, or dig out the bucket every five cycles before it compacts too tight to get it out easily.