Meta ads high CPL low conv

What is meta ads? Tried advantage+, optimized creatives, ad placements, location, age, day parting. Regardless of anything and everything, the performance rises in 3 weeks of launch and then plummets steeply, if I copy paste the same campaign then the newly copied and launched campaign works well for 3 weeks and dies again. PLEASE recommend what can I do here to have it run efficiiently? PS ads for a Healthcare Clinic