But did they look?
If they found a bullet a few feet from the crime scene that was never fired, was not the murder weapon, and had no blood or dna or fingerprints on it, the obvious question to ask is "How unusual is that?"
The obvious investigative step is to run a metal detector over the adjoining area (and beyond just the crime scene). If that was the only bullet, it makes more sense to presume it may have been related to the crime. Especially since there was presumably no independent evidence that any gun was involved in the crime - the girls saying "gun" is suggestive but could have been expressing a subjective fear (like when they said he was creepy).
If there were 1000 bullets of the same type (this is the USA) then that one being there says very little. If there were 1000 and 50 seemed to have marks that could have matched RAs gun, then the one bullet is all but meaningless and not even probable cause IMO.
Doing a thorough investigation rather than assuming things is good practice. There were two other suspects RL and KAK that were suspected based on circumstantial links to the crime scene and/or communications with the girls. RA is the latest, but it has to be proven.