ATM’s in India are a bit variable. Rejected at most in Delhi airport except Citibank ATM.
Traveling near Lucknow card rejected at Union Bank of India but accepted by Allahabad Bank. Some ATMs had no money and some were rejecting a few local cards also.
When card worked got very good Rupee rate although some ATMs have small local charge (such as Citibank ATM in Delhi Airport).
Please note you get a lot of Rupees per GBP and the cost of living is not high so you don’t need much cash in India. I suggest small amounts and frequently or else you get very high denomination notes !
Starling say will arrange for Mastercard database to be updated so non-functioning ATMs should come on-line in due course.