Jennie, hi! I have YOPD and I've been on Madopar for one year now. Noticed improvement in my gait with the first pill (one quarter of a pill, 62.5mg). Many patients with PD will tell you a similar story, i.e. that they responded almost immediately to levodopa. However, not ALL patients respond this way. For example, patients with YOPD tend to have a more "robust" response to levodopa and at lower doses, as compared to patients with late onset PD. Furthermore, in patients with long-standing PD, more advanced disease stage or other comorbidities, it may take several weeks or even MONTHS before the full benefit of levodopa is seen, through SLOW titration to the appropriate dosage, which is different in each patient. I'm actually surprised your doctor agreed with discontinuation of levodopa that early on. That said, there's always the possibility of an alternative diagnosis, especially if symptoms remain unresponsive to relatively high doses of levodopa after a period of several weeks, but, in my humble opinion, I don't think you are even close to this point. You could also ask for a second opinion once you have the brain MRI results.
Good luck and keep us posted !!