Without knowing anything about your process, I can offer only a few insights: Diacetyl from yeast is usually created during the early stages of fermentation, it can be cleared up using a diacetyl rest which is usually appx. 10 degrees F above the fermentation temperature. It can also be caused from infection, sometimes from unclean serving lines etc.
Here is what palmer says regarding diacetyl:
"Diacetyl is most often described as a butter or butterscotch flavor. Smell an unpopped bag of butter flavor microwave popcorn for a good example. It is desired to a degree in many ales, but in some styles (mainly lagers) and circumstances it is unwanted and may even take on rancid overtones. Diacetyl can be the result of the normal fermentation process or the result of a bacterial infection. Diacetyl is produced early in the fermentation cycle by the yeast and is gradually reassimilated towards the end of the fermentation. A brew that experiences a long lag time due to weak yeast or insufficient aeration will produce a lot of diacetyl before the main fermentation begins. In this case there is often more diacetyl than the yeast can consume at the end of fermentation and it can dominate the flavor of the beer."