Manually (if possible) check the bowl is closing and opening correctly. If not, check the bowl opening water supply and bowl seals. 

Check for scale build up in operating water passages. Also ensure operating water is getting to the separator.

Check the oil operating temperature is correct and flow to the separator is correct. Check the inline filter is not blocked.  

Stop the separator and check the bowl has been correctly assembled and clean sludge deposits out the bowl. 

Release Brake. Clutch pads worn or contaminated and may need changing. Bowl is open or leaking. Check bowl.

Check gearbox oil is correct level. Check gear box for damage. Check Separator spindle height is correct, especially after servicing

Cleaning is nearly impossible. Just order a new one. Normal working life is around 3 years. 

Check the tightening dimension in the heaters manual/spec sheet. Tighten evenly, do not over tighten. Order a set of gaskets

You will have to remove the cracked plate and one other.  Re-tighten to the new tightening dimension.