Presumably, they at least took a report which could make it possible to file an insurance claim for the stolen phone.
But, a good faith belief of law enforcement that is unable to figure out who committed the crime with the resources available to them is a legitimate reason not to investigate and prosecute a crime, even in countries with legal systems based upon the German one where prosecutors have an enforceable legal obligation to prosecute criminal offenses known to them. A prosecutor can't bring a case unless he knows who did it. And, the police may be wise not to try to investigate a crime that previous experience has proven to them is a dead end most of the time for a crime of modest economic value.
Most cases of simple larceny are never solved. And, institutionally, the police have to balance the cost of investigating the crime against the seriousness of the crime. Murders and kidnappings are almost always going to take priority for police resources over stolen phones, particularly if investigating the phone theft may require international cooperation that makes the investigation more costly.
A key point is that the mere fact that a phone is pawned doesn't mean that the person pawning it is necessarily committing a crime, so even if you find out where the phone is in some pawn shop (possibly in Albania or Turkey, by now), that doesn't mean that the job is over. That person could have bought it innocently and at fair market value from a flea market, from someone who could have gotten it from someone who received it innocently as a gift, from a friend of a criminal fence, who may have bought it not innocently from a true thief. In fact, because criminals respond to incentives like anyone else, it is far more likely that the phone was swiftly "laundered" along the lines I suggest in this example, than it is that it is still in the possession of someone close to the thief.
But, only the fence and the thief would have criminal liability.
It is a lot harder to solve the crime of a stolen cell phone than you would think, and the agency may simply not be able to justify the resources it would take to investigate that case properly to a situation where someone had a phone stole that was worth maybe 400 Euros in used condition.
The more you can do to solve the crime, the more likely it is that they will find that it is worth further investigating until a thief can be identified and apprehended. For example, the company with the network that serves the phone might be willing to cooperate with its owner. Also, the more that you can do to show that this case might crack an entire ring of people involved in a black market in stolen cell phones might make it more attractive to law enforcement.