Thumbnail Personalization Results Keep Getting Worse

When I started using Thumbnail Personalization for Titan Tower Defense the results were overall pretty good. Most thumbnails did not fall below a 1% QPTR. However since then the results kept getting worse with each time I would try new thumbnails even though some of the thumbnails were literally the same and the QPTR they were getting was 3x less. Besides QPTR, session time also drastically changed from 11 minutes to around 5.