A couple things come to mind
This is where I would look first for consistent coke drum cleaning, is your cutting procedure current, is every coke cutter following the (current) cutting procedure?
If the coke drum cutting techniques are all over the map there could be coke left in the drum post cut. When the coke drum is considered empty does the decoker do a couple clean out passes to ensure there is no coke on the shell of the drum?
Is your jet pump providing enough pressure to impact the side walls of the coke drum to make sure all the coke is removed?
Be sure your Unibolt or flanged stem to too cutting tool connection has no leaks.
Do you have a dome clean out procedure to remove the coke above the in-drum level of the coke drum this should be done as a routine duty weekly for each coke drum?
Most 1990 and newer cokers have this dome clean out ability built in through a key lock system. Older cokers please review and build a safe procedure to do this task.
How do you make sure there is no coke in the coke drum for internal scaffolding for inspection and coke drum crack repair?
If you have coke left after cutting does this affect your coke drum warm up? As the coke drum expands during warm up coke will fall off the side walls plugging up the warm up lines.
I was on the un-heading deck watching a contractor do some maintenance work on the Hahn & Clay un-heading device when a 200+ hundred pound coke chunk fell out of the coke drum He spent some time in the hospital I dont know if he ever worked again.
More questions or if you want to discuss this give me a call at 360-758-3073 Gary