I have waited for the ones that make yogurt all the time to have their say and I have this to add, either buy a yogurt starter or be sure that the purchased yogurt has a LIVE curture in the cup.[many of the big producers are now making extra health claims by using different bacteria to make their products and I am not sure if the FDA is requiring that the cultures be killed before packaging or if the companies are doing it themselves so that others can't profit from their R&D.] Just about everyone lists culture in the ingredient statement but if a seperate statement along thia line [this package contains LIVE culture] does not appear then it is unlikely that it will work as a starter in your kitchen.
Second temperature control is important. If you add your starter to milk that is too hot it will be toast, killed by the heat. Too cold and it will be a very slow grower because heat controls growth, too cool and it will be thin and watery in the time you allowed, you could try more time and see if it thickens. Too much time in a too warm space and it is likely to be too tart. [when trying new conditions we would sample often and knew that refrigeration only slowed the growth of tartness.
To sum up a new maker will not help a dead starter but with better temp and time control it might help with texture and tartness problems. ~gd