If the yoghurt has not set properly, it may be either that the temperature was too high when you added the starter (needs to be about 100-110ºF) or the starter may have been ineffective for some reason.
If you are using commercial yoghurt as your starter, use one that is as fresh as possible. Also, as Pene says, use one that is "plain" - no gelatine or other additives.

Are you able to maintain the temperature between 100 and 110º while it is culturing?

After I have finished culturing mine (for 24 hours, because I follow SCD. The longer time eliminates most of the lactose - making it lower carb although a bit more tart), I place it in the fridge for at least 8 hours without disturbing it. It seems to "set" more in that time.

You can also "drip" your yoghurt to make it firmer. This just means draining it through cheesecloth or a coffee filter. The whey that drains off also carries off more carbs.
I do this, spooning the yoghurt into a coffee filter in a strainer over a dish. I just leave it in the fridge for about another 8 hours or more.

HTH!

Roz