A few things need to be considered:
The starter culture health: Ensure the starter culture 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 starter culture activity declines. It should still set, but may take much longer.
The right milk: Ensure the correct milk has been used, as specified by the recipe.
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.