There are four possibilities:

1. A different building is still being upgraded. You need to wait until it is finished, use gold to finish it instantly or hire a new builder.

 

2. You are missing some resources to upgrade the building .If that is the case, you can still upgrade using gold or buy the needed resources in the shop.

 

3. Your upgrade requires another building or research. Do the required action first to proceed with the wanted building.

 

4. You have reached the maximum level of the building.