I cannot say for sure why your yogurt didn't thicken, but have noticed that, apart from starter quality and milk temperature, goat diet can be to blame on inconsistencies in all cheese/cultured milk products. I believe that GMO grains change the behaviour of proteins during cooking and culturing. If you feed your own goats, avoid all GMO grains and see what happens. Besides this factor, be sure that all measurements are reasonably accurate, and try heating your milk to blood temperature and not much past. Beyond about 115 degrees F., many live organisms begin to die. Be sure your starter hasn't been contaminated by sitting out, changing temperatures, or being subjected to other conditions that could tamper with the desirable cultures. Lastly, yogurt made from other batches of yogurt which are too many "generations" from the original starter may fail to work. To avoid this, you may freeze small batches of a fresh starter so that you can stay with second or third generation yogurt for some while.