A few things need to be considered:
The rennet health: Ensure the rennet has been stored correctly, and is not past its expiry date.
Time and temperature challenge: If you were unable to maintain a nice warm temperature over a time period, as specified by the recipe, the rennet activity declines. It should still set, but may take much longer.
Non-chlorinated water: Did you dilute the rennet in tap water? Rennet must always be diluted in cool, non-chlorinated water. At home you can use cooled, boiled water – as the chlorine is lost during the boiling phase.
Disturbing the curd formation: You need to ensure the milk is completely still/undisturbed during the setting time. Movement of the milk will disrupt the curd formation.
The right milk: Ensure the correct milk has been used, as specified by the recipe.