Method with @Activate annotation will only be called once your Component(or Service) is activated. If Component has any dependencies it won't be activated hence your activate method won't be executed. Check if your Component/Service has any dependencies and try to resolve.
There is no such constraint
Could you please go to systsem console and inder components tab go to your service and see if it is axtive or not not some refrences are in satisfied state.
If its not active it will not work, try to resolve dependencies.
Hope this helps
Please check logs for any errors and paste the code here