I'd buy new starter.
The old stuff will have wild bacteria in it.

That's weird. I can use my own yogurt for the next batch around six times before I need a fresh batch to start with.
I can't think of why this wouldn't happen for you.
Did you start with yogurt, or a powdered starter?

As for buying new starter each time, I thought I'd share what I do:
I buy a quart of good yogurt and use a few spoonfuls to start my own raw/local batch.
The rest of the quart gets frozen in ice cube trays so I have some on hand the next time I need it.
Once I have cubes, I put them into sealed bags. I use two cubes to make a half-gallon of yogurt.

It's so cool how the cultures can survive the cold. They just seem to hibernate until we bring them up to mammal temperature.
What a nice coincidence for us all. :)

elevation will effect your cultures. this could be the variable determining how often u need a fresh starter. When I traveled through the Andes in South America, all the yogurt was very runny due to the high elevation.

Thanks for the ice test tip!

do you heat your milk?You are suppose to heat your milk first just to the point before it gets too hot to put your finger in it. I forget the temp actually because I use the old fashioned finger in the pot method. Then you add your yogurt starter and allow it to ferment for 8 hours in a warm place. The longer you let it sit the more tart it gets so if you are leaving it over 8 hours that may be part of it.