Early on the police said everyone was cooperating. Now, we're hearing this person or that person are not cooperating. I suspect the parants, LE, PI's, etc. want more bites at the apple. Since these people didn't tell them what they want to hear or anything of significance then they want to keep asking questions until they do. Otherwise, they are not cooperating in their eyes.
I understand the parents' frustration but then I can also understand how they could also be in some desperation too.
If any of CR, JR, and MB are really the police's chief suspects then they must've been good at avoiding leaving any evidence that morning. Good enough that the police feel they need a body to prove a crime. From the police's actions it doesn't appear they even did anything that would call their alibis into question (like a cell ping from somewhere when they claim to be elsewhere). At least not enough for the police and/or prosecutor to act upon it.
And what of the alleged mystery guest at JR's? He could infact be a 4th alibi witness corroborating the story of the other 3. Doesn't mean he's telling the truth of course, but it would mean there's one more person needed to keep a story straight and not have any loose ends.... or else one more person who tells a story that dovetails with the other 3's story.