why has my percentage of proficiency dropped from 55% to 47%? I practice Spanish 4-5 days a week.
Just ignore the proficiency. It doesn't really add anything or reflect your capabilities. You can have 55% yet you're only 1/3 of the way through your tree or 30% yet your tree is complete (all gold).
There are lots of threads that try to explain it but I've never seen a formal explanation from Duolingo on how it really works. The highest I've ever manage to get it is 65% and it stayed there for only one day, then it started to go down. From what I remember it's a combination of practice , strength of each word, number of words etc. Unfortunately the practice session does not refresh all weak words just some no matter how many times you click it. I wish they had a feature that would practice only the weak words.