Assuming you had plateaued on sema (which wasn't specified, but I'm guessing that could have been the motivation for making the change to tirz), then tirz is "working" in the sense that it (along with your other changes) has kept you from gaining weight back. It's just not working strong enough to lead to additional weight loss, it would seem.
One thing that I think isn't properly appreciated about GLPs is that although there's a calorie reduction aspect to them, that's not really the full story when it comes to weight loss. I found this to be an interesting blog post, as it does a good job breaking down a really interesting GLP rodent study that informs on this topic:
https://uncertaintyprinciples.suspicious-link-removed.com/p/why-do-we-lose-weight-on-glp-1-drugs
Set aside the fat partitioning and respiratory quotient stuff (where I think Gary may be making some faulty inferences) and just note the study result itself. Specifically, that the matched calorie control group lost less weight than the GLP group (despite having identical calorie consumption).
I can't even begin to guess at the biochemistry going on in your body, other than to note that if it's only been several weeks since you titrated on up tirz, that (due to the half-life of sema and tirz), your GLP-1 to GIP agonism ratio in your body isn't at normal tirz ratios yet and is still leaning more strongly towards GLP-1 agonism (from the residual sema). I can't speculate on whether that's a good or a bad thing, but I can confidently assert that you're not yet at the steady-state GLP-1 to GIP ratio that you'll be at a month from now (assuming you stick with tirz).
As a practical answer to your dilemma, some people where you're at currently will add in maz or survo (the "ghetto reta" method), which (perhaps negatively) tips the scales back towards GLP-1 agonism being stronger, but also flips whatever metabolic switch glucagon agonism is hitting. I have no idea if such an approach is superior to reta, other than to note that people (especially those coming from a sema background) seem to feel more comfortable with the style of appetite suppression that comes from a stronger GLP-1 input than reta on its own delivers. Which is more effective in terms of results? I have no idea!