Nandini Milk — Nandini milk is not curding properly

We have been buying Nandini milk for 40 years and made good thick curd from it so far. Since 20 days Nandini milk is not curding properly. We put sour curd (Heppu) to Nandini milk from all the possible sources including cow milk curd, Namdari curd etc. Nandini milk makes watery, stale curd. Sometimes with bad smell. However with the same Heppu, other milks produce thick good curd. If this issue continues we will be forced to buy other brand milk.