Mozzarella not setting is my milk good

[nettie]

Hi All I am hoping you can help.  I am currently making mozzarella with a good stretch from cows milk.  I have had the odd batch in the last week where Ihave  had no strech and even the curds are not setting together when I heat them up to stretch.  I am trying to find out what might be going wrong.   I am using raw un pasturised milk as I have done everyday and the same culture and rennet. The milk is from our farm.  The milk when heated seems ok but there is a slight yeast like smell to it,  and although the curds on cutting seem good when it comes time to heat them up for stretching they do not mix and there is no stretch at all.  I have also noticed that the PH levels are sometimes increasing.  Does anyone have any ideas on how I can fix this.
I thank you in advance for all of your help.  Cheers Annette

[AnnDee]

How do you acidify the milk?