First of all - my sympathies to the tester who didn’t feel like they were advancing any more. I was in a scenario where I was technically doing my job, but I didn’t like it, I wasn’t being challenged, and therefore was definitely not thriving. It is not fun to be there.
When I was in that situation, I couldn’t take the monotony of just doing my job. So, I honestly started volunteering myself to do and help more. I would help coworker Y with running their manual test cases, I would offer to document coworker X’s automation code reviews, I started helping my project manager write specifications, I read the entire manual on the version control system we were using and could now be technical support for problems my coworkers came across, etc.
These little things added up to a strong driving force towards learning more, broadening my skill set, increasing the visibility of my value to my team, and helped me gain confidence for my biggest leap - I proposed to do all the manual and automated testing (a brand new skill I didn’t then have) for a new project. That was when I discovered that I truly am a programmer at heart. All things automation provided a never-ending stream of discovery, advancement, and improvement like Amy and Steve have both mentioned.
I feel like the analogy of a snowball getting bigger as it rolls down a hill is apt for describing my job satisfaction. The more I customized my job description, the happier I was. Now, I am in a very enjoyable and meaningful position, albeit with a very hard to pin down job title.
So my word of advise is that it could be worth rethinking what “advancing” means and how it can be attained. For me, my managers weren’t handing me advancement objectives and weren’t giving me opportunities to stretch myself. So - I redefined “advancement” as “doing something I wasn’t doing before” and allowed me to start finding opportunities for my “advancement” everywhere around me.
It is worth noting that promotions and career ladder advancements started falling into place for me even though I wasn’t following any prescribed career path.
Best of luck!