If the process was the same, was the processing time longer. Longer processing times give you thicker product, but it is more sour.
The only other possibility I can think of is that your yogurt post has some small bacterial contamination. I usually put 5-6 mm of water in the bottom of the cleaned pot, and then microwave the pot and lid on full power for 4-5 minutes. This is time enough to boil the water and get maybe another 3 minutes of sterilizing steaming. I do this immediately prior to using.
Please do not microwave metal pots!
Yoghurt cultures are a combination of various bacteria that turn milk into yoghurt.
Nowadays we are usually used to the combination of streptococcus thermophilus and lactobacillus bulgaricus. This gives a rather mild only slightly acidic yoghurt.
If lactobacillus acidophilus is used then one tends to get a more acidic thus sour yoghurt.
Lactobacillus acidophilus is a very common bacteria that lives on your hands and in your mouth and in your guts. It is a very strong bacteria compared to the other two and can dominate them. So if you use yoghurt over and over again as a starter there is a real change that lactobacillus acidophilus takes over and you get more acidic yoghurt. Contamination of your culture can as well occur when your yoghurt to be comes in contact with the lactobacillus acidophilus living on you. (You can just spit into milk and make yoghurt that way). Avoid using old yoghurt as starter culture when this yoghurt has been contaminated for instance by a spoon that has been in your mouth.
As tiger mentioned above a higher level of hygiene can avoid this or using fresh starter culture with only streptococcus thermophilus and lactobacillus bulgaricus.
Luckily my yogurt pots are not metal.
Starting from powdered culture bought in JD, I have let replicating the same yogurt for one and a half month now. It is only very slightly more sour than at the beginning, which is actually better to my taste. Key is to keep the borders of the pot clean. It's in the borders that the yogurt goes bad adding acidity to the whole. The pot should also be kept well covered, both in the fridge or outside when making it.
These powdered cultures work fine.
I use powdered culture as well and then use the actual yogurt to start new batches. Even after more than six generations I didn’t notice any difference in the end result. Mind you I use individual jars of 200 ml thus limiting in a big way the change of contamination compared to making 1 litre batches and using a bit of them.
For powdered culture see:
See as well the thread:
On the same topic.