At the begining very project is wonderful, but sometimes things start to slip, so when do you pull the plug?

Budget! When activities are made with more of the predicted expenses.
Other important thing is when the team is not functioning as at the start.
Consultation with key stakeholders is needed as well as constructive meeting with the team.

Go back to drawing board.
re examine the purpose of the project.
See where you are right now.
check if you are expected to give more from its current point forward then recheck if what you have to give in will still meet your purpose.
Every project has its purpose and its goal.
When you are not going to meet that goal, you are getting more losses than gain and you are seeing you will get more losses to continue on, then its time to pull the plug.
But take note to check its long term benefit against its losses.
some projects are late bloomer and you nee to know if your project is one of those.