Cartier that can be a trickly question, and I think it will depend on the cheese that you are making. I have found that you have a few options.
1. If the set is really poor, and the milk still fairly liquid, I have at times added some more rennet. This of course is assuming that you know that the set problem is not because or old/out of date rennet.
Sometimes, if the water that we dilute the rennet in is too hot, it renders the rennet useless. So if after the second addition there is still no set, I cut my losses and give it to the animals. There have been a couple of occassions where I have no other explaination other than milk quality being the problem.
2. You could change track and find a cheese that calls for a lesser rennet use, and make it. The fetta recipe that I use, calls for a lesser rennet usage (1.6ml per 10ltr rather than the usual 2.5ml per 10ltr) and a setting time of 1 - 1 1/2 hours. That is if the culture that you used is suitable for that.
3. Depending on the set, say at least a medium set, you can proceed, understanding that you are not going to have the larger curds, and that the curds will shatter very easily. So you can stir occassionally to minimise shattering the curds. Draining off the whey probably will require the use of a cheesecloth, and the resultant curd then scooped into moulds.
Usually I find that I can get a cheese in the end, but with a reduced yield that is drier and a little more brittle than usual. The whey usually makes a good ricotta.
I also would probably leave it set for a maximum of 2 hours, and that would depend on the room temp. Remember I live in the tropics. If it is not set by then, I don't think it will set any more with more time.
Calcium can help with the setting of milk too, but I think I remember you saying that you use it anyway.
Hopefully someone else will be able to add to this, but this has been my experiences.