There are a couple of ways to look at this question. The first is - why do you think you are failing? Do you have an annual performance review and a chance to discuss how to expand your role or profile? Does the Teaching Fellow have a pedagogy focused promotion track where you are? How are you defining success/is success being defined where you are?
The second way to answer it is to offer some reflections of my colleagues who are Teaching Fellow equivalent (RG Uni, social sciences). They tend to fall into two camps - those who wish to switch to a research and teaching role, and those who wish to develop on the pedagogy side. So where I am, there is a Learning and Teaching conference and funding available for pedagogy research. These can be projects to improve student experience, creating new learning resources, or develop cross-disciplinary initiatives, these are just a couple of examples I know. They can also lead to publications. There is not an expectation to publish at the teaching fellow level, but there would be for promotion.
if you are not aiming for promotion, then there are many other things which teaching fellows have done where I am, which have really improved the teaching culture in my area. Things like regular group sessions for graduate teaching assistants to discuss pedagogy and issues arising in the classroom, making additional teaching resources in dialogue with the lecturer, offering support and ideas for the virtual learning environment, being innovative with technology and then sharing their practice, so many things which lecturers might not get to because they are so busy. But that said, these staff have since been promoted to the next level because they are looking for development.
This brings me back to the context of universities being quite hierarchical and the culture that you will be working towards promotion. If that is not what you want, then I do think it is important to spend some time thinking about what you do want, and how you want to place yourself in your institution and then own that. You say ‘if they will have me’. Are you on a rolling contract? Is it possible for that to be open-ended? I think you need a bit more of a sense of your own worth here. Four years of experience is not a trifle. You will have experience and insights to offer to graduate teaching assistants and new teaching fellows, whilst also being able to provide ‘coal face’ experience to lecturing staff of how to improve their teaching materials for delivery.
I suppose if I were your line manager, I would be trying to understand what you do want out of your job and development, and how to support you to achieve that.