Sure, breakthroughs definitely happen.
Like you I was on 20mg Amitriptyline and had no major attacks for 6 months, lulling me into a false sense of ‘security’. Then I was hit with an almighty attack that really knocked my confidence. Subsequently when an attack hit, I’d have 2 especially bad days and then a slow recovery back to baseline for 2 weeks or so.
These pills are symptom control and not 100% foolproof. They are unfortunately not a magic bullet. That said, Amitriptyline was a massive help. Make sure you are also paying attention to the 6 C’s. Sometimes, being on medication, one can lose discipline with other things. Caffeine is a huge trigger, so avoid at all costs.
But of course, my condition improved over years in any case and all those major attacks are history. Hang in there!