Crime Lab......Again

Last night I drove down to Rockaway Beach to pick up the pickup so my daughter and her husband could move the horse. On the way back I was listening to KTTS news and heard, I think, the Christian County Prosecutor talking about charges not being filed against one of the suspects who murdered this woman and hid her body in the Mark Twain Forest.
The prosecutor said that evidence from the crime lab was not forthcoming in a timely manner. In essence he was told by the crime lab people, 'What homicide do you want us to not investigate while we investigate your homicide.', or words to that effect.
The prosecutor than said that the Jackie Johns case was the exception to the rule. From the probable cause statement: 'On August 8, 2007, I executed a search warrant on Gerald L. Carnahan and obtained several bucchal cell swabs from the body of Carnahan. I then transported these bucchal cell swabs to the Missouri State Highway Patrol Crime Laboratory in Jefferson City, Missouri. The crime laboratory then obtained a DNA profile for Carnahan from the bucchal cell swabs and compared this profile to the unknown profile obtained from the Jackie Johns vaginal swab. Upon comparing the two (2) samples the crime laboratory advised that Carnahan's DNA profile matched the unknown profile obtained from Jackie Johns."
-- Sergeant Daniel F. Nash, Missouri State Trooper"

How about that murder in Hollister? The DNA results came back in less than two weeks.

How come some cases get done in less than two weeks and some don't get done at all?
Who makes these decisions and what is the basis for making them?