No. It just means that they did not have enough evidence. Many cold cases prove your claim wrong. For instance a case of Maria Ridulph, kidnapped from her home in 1957, her decomposing remaind found the next yoear. The police had their suspect pintpinted from the get go. Unfortunately the dude, provided the alibi that was backed up by the witnesses. There was not enough evidence to bring the case to court, it got shelved for years. It was reopened in 2011, the new evidence was found after the exhumation of Maria's body and the new witnesses stepped in, including a woman, who was Tessier's girlfriend in 1957. She helped to demolish his alibi. And voila, the man who was the suspect number one from the very beginning got sentenced for this murder. So it seems the police theory was not flawed in spite of the lacking evidence.
Hmm. Interesting case, there is some serious doubt about the safety of the conviction - in fact as with the Madeleine McCann case there were no clues left when the abductor struck.
Also interestingly there was no agreement regarding the timeline - does this sound familiar and does it confer suspicion on those with the conflicting stories?
"Already, there were conflicting reports about the exact time of Maria's disappearance. Was she snatched closer to 6 p.m.? Or did it happen later, at about 7? Police and FBI reports, as well as news accounts from the time, contain details that support both scenarios.
Sycamore's police chief, William Hindenburg, told FBI agents that Kathy and Maria went out to play at 6:02 p.m., but the DeKalb County sheriff said Maria didn't call Kathy and ask her to come out and play until 6:30. Maria's mother later altered her original estimate, saying the girls could have been outside as early as 10 minutes to 6".