There can be a few explanations to that:
- For the expansions to the left of the Railway Station, you need to complete its building
- For the expansions to the right of the Railway Station, you need to complete building the Airport
- For the expansions beyond the Bridge, you need to buy both bridge expansions on either side of the river
- For other expansions, you need to reach the required level
If there is no more land left to buy in the City Center, you are free to expand in other areas of the map.