You may be doing something completely right! As your impressions increase YouTube is naturally going to share your content out to more and more viewers and not all of them are going to be interested in your work... so the more you get shared out the lower your CTR will inevitably be.
That having been said it is also possible that you simply need to continue testing thumbnails/titles. There are an obvious number of things that could cause a thumbnail to perform poorly with audiences. They do not correlate though; consider each thumbnail as a standalone and judge it on it's own merit rather than judging a series of of thumbnails and their performance together.