This is just plain really bad police work. What good is a database if you don't use it?
If you can't find the car, either he is:
(A) not getting stopped at your roadblock, or
(B) being stopped, but being let go.
There just aren't any other possibilities. To check (B) you just search your database for cars with multiple stops. What are the odds that the same car would be at more than one shooting site? How about more than two?
By the third roadblock stop for the same car this thing should have been wrapped up.
And, by the way, this is not exactly rocket science. A perfect example is that this is the way they finally caught "Son of Sam". Thye New York Police searched their ticket database and after exhaustive looking finally got him through a parking ticket near one of the shootings.
That job was a lot harder, they had to sort throught lots of tickets. In this instance they had the example of how to do it, and a large number of roadblocks, where clearly license numbers had been recorded.