If one yoghurt pot doesn't set don't thow it away. Start again with the unset pot add fresh boiling water up to the limit....likely the water wasn't hot enough to start with.
I had these from IW in the past and found more did not set than those that set.
I tried hot water in the mix and boiling in the tub. Even replaced the water 12 hours later when it had not set.
No luck, I even put half the water in the pot with the culture again no joy.
I was told on this web site about http://www.yoghurtdirect.co.uk and have not had a problem since using them.
I can only put it down to the quality of the product supplied. To get the price lower quality goes out the window.