You are unfortunately in that proverbial area between the rock and the hard place. Unfortunately Law Enforcement has to work within the law. That means that what YOU know and what YOU heard are really not of a whole lot of use to them except to have them aware. They cannot make an arrest based on “he heard”, or “he knows” or if they do, they are fully aware that it is not going to make it through the courts and one must remember that once acquitted of a charge, they cannot be re-tried on that charge.
They went out, talked, looked and unfortunately found no hard evidence. But LEO do not easily forget things like this and they will be keeping a quite eye on your potential perpetrator in hopes of find real evidence in your case or another … at least now they have someone to keep an eye on.
As far as helping you to recover YOUR stolen property … unfortunately that is where the hard place comes in. It is unlikely that you are going to see you bikes again so hopefully you had insurance. Yeah, we all wish the real world was like where the cops would grab the guy and say we know you took them, and we are going to beat it out of you what you did with them … but that is not the real world because the bad guy has rights too, and often free lawyers.
Complaining about how LE did not do their job is not going to get you anywhere and will only make it harder to deal with them later. Keep your eyes open and if any real evidence of this bad guy doing a crime … yours or another … then try to get the LEO involved as fast as possible. In general LE does a very good job in what is a tough environment working with hands tied .