Does finasteride ever just stop working or slowly lose effectiveness?

Finasteride is effective in about 95% of patients. It takes a bit of time to see the effects as you may have noticed (no less than 6-12 months) and then the effect "peaks" around 24 month and stabilizes. Finasteride does NOT regrow lost follicles although you may have felt like it did for you. Instead what it does is a) keeps hair in the growth phase longer and b) encourages hair follicles that have miniaturized to cycle back toward thicker, more substantial "terminal" hairs. This second effect is why people think they regrew hair. In fact, those follicles were still there, they were just producing very small, pale hairs and when those hairs became more substantial and noticeable we think they are "new" . So at this point, the finasteride is probably still working perfectly fine but is just at it's peak effect and will keep your hair in it's current state for the foreseeable future, which is frankly, what we expect and the best result we can ask for. Any further "improvement" would need to be via FUE/FUT transplant most likely.
On occasion, I see a patient for whom finasteride loses it's effectiveness after 15-20 years and sometimes switching them to dutasteride helps resolve this issue. But in those cases, it's an obvious re-initiation of hair LOSS. I hope this answers your question.