Your project scope is probably impossible to control because you seem like the kind of project managers who can't say know to their client and/or to their stakeholders. You process all change requests and you don't integrate these change requests properly in your project schedule. You don't re-baseline the project when you have to. You also don't know how to gather the proper requirements.
My advice to you is:
- Gather the right requirements from the first time: Ask your client a lot of questions about his business, understand his business, ask him a lot of questions on what he wants from the product.
- Try to say "No" more often: I know that the idea of saying "No" to people higher than you in the food chain is a bit intimidating, but trust me, they will respect you more for this and you won't get fired.
- Formalize change requests: Controlling changes starts with making changes formal. For example, people have to sign papers that they want a change to be done, and a (substantial) change needs to be approved by the CCB (Change Control Board).
- Negotiate change requests: Not every change request needs to be processed immediately. Maybe you can convince the client that some change requests need not be done, while other changes can be postponed to Phase II of the project.
- Update the schedule to reflect the changes: Update your schedule immediately when you integrate a change request. You should re-baseline your whole project if you have integrated a lot of change requests.