Are you diluting your rennet in 1/4 cup cool water before adding it to the milk? Then stirring it thoroughly for about 20-30 seconds, use your skimmer to stop the motion of the milk, then leave it to set up?
The first question is because it could be due to a lack of the rennet being able to get into all the milk. The second question is in case you are perhaps overstirring it or the milk is being disturbed after the rennet has been added.
One member reported that they were setting the kettle of milk on their chest freezer (to get it out of the way) while it was setting up and they never got proper curd set because of the freezer turning on and off.