We want the police in the real world to be like those we see on TV, but they are not. Crimes are not solved quickly with the correct person brought swiftly to justice. In reality the police are poorly resourced, under trained, and sometimes just plain incompetent. Not everyone can be Sherlock Holmes.
The wrong person is sometimes brought to justice, or not all. Sometimes crimes are not even investigated due to a lack of resources, or even interest. Decisions may be made not to investigate a crime due to statistics. For example the likelihood of it being solved or someone being brought to justice, the time required to prosecute in court, and the amount of tax-payers money required.
There is a huge difference between reality and TV when it comes to law enforcement.