This has been a persistent problem for me. It is caused by a lack of proper lactic acid formation in the milk as it cultures. This can be caused by:

* The type / brand of milk, its age, whether it has been homogenized, pasteurized, etc.

* Poor culture quality that does not create adequate acid formation

* Wrong temperatures

* Etc.

When using store-bought milk that has been homogenized and pasteurized, several experts have recommended the use of calcium chloride (CaCl) additive to help create firm curd. This compensates for the loss of natural calcium in the pasteurization process.

Look at the quantity of rennet called for in the recipe. Add this same quantity of CaCl to 1/4 cup of cool bottled water, stir well, then stir into the milk well prior to adding the rennet.

Also, make sure the pot of mik is not disturbed in any way during the renneting process. Cover the pot to prevent drafts. Do not bump it or move it. Disturbing the milk during the renneting process can compromise curd formation.

Make sure the milk is at the proper temperature before adding the rennet. And, during the renneting process. Use a water bath to ensure a consistent temperature over the time required (usually less than 1 hour).

Stir the rennet in well by stirring milk up from the bottom of the pot toward the top of the pot. Use about 20 strokes—slow and gentle—but do not stir for more than 4 minutes.

Finally, if none of this works, you might consider increasing the amount of rennet by a small amount.