You have to make each round using yogurt from the previous round, not the preceding ones (1), else the bacteria in the culture probably get too few / weaker due to less than optimal conditions, like temp., hours out, etc. (1) That is, batch 2 using starter from 1, 3 from 2, etc.

E.g. google videos of how to make kefir and see what they say about how to preserve and replicate the kefir "grains".

Exact same technique may not apply to yogurt culture, but principles do.