Yogurt has stopped thickening properly
That happens sometimes, it means that the ecological balance in your yogurt has shifted to some less tasty bacterial strains. There is even a small chance that you picked up something slightly pathogenic.
There is neither a need nor a practical way to shift it back to the original strain. It is best to start with a new, known-good culture. You can get either store-bought yogurt, or pure starter from a type you enjoy.
You can expect this to happen again and again after a long chain of reinocculations, because in home conditions, you don't have perfect control over your fermentation process. It is just part of the life of a yogurt maker to get back to a fresh culture when the homegrown one weakens.