It is most unlikely to be a problem with the rennet. I’ve had a number of similar enquires this year, and it appears to be a milk issue. Milk proteins are very variable, and this season appears to be more than usually fragile in some cases. I suggest you try buying skimmed milk and double cream, instead of whole milk. Combine them at the ratio of 12 parts skim to 1 part double cream. I’m using this myself at the moment, and with a supermarket supply; it’s going OK.