Quality Score is based primarily on CTR as compared to your competitors. If your CTR is low, as compared to your competitors, then your QS will be poor. If your QS started higher and then dropped, a relatively low CTR is the likely cause.
The way to fix that issue is to delete any keywords that have really poor QS. Then pick your best performing keywords, write very targeted and compelling ads for the exact match the only version of the keywords. Anything that doesn’t get relatively high CTR should be immediately removed and replaced with more compelling ads.