OpenTTD City size declining

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I had a city with 25000 citizens, but it started to decline. As you can see on the screenshot there are a lot of empty spaces. There used to be building along those roads.
To prevent the decline I added bus routes within the city. According to the OpenTTD wiki that's a way to increase town sizes:
A bus with one passenger from the other side of town would have the same effect on growth as a large train full of passengers from the other end of the map. source: wiki.openttd.org
But the city still shrinks. Does somebody know how to prevent this?
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How do I expand my city in Openttd?
Town growth can be accelerated by loading or unloading at least one item of cargo at up to five stations within town influence within a 50 day period. It does not matter which cargos are loaded/unloaded.How To Grow Cities (OpenTTD Game Mechanics 04)
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