[abtom]

In my experience it's usually the bank having an issue on their end. If you try to make a transaction late at night, say 2am, there's a good chance your bank is "down for maintenance". My account is with a public bank, not sure if this is true for private banks. Either way, reliability varies from bank to bank.

One reason behind this (communicated to me by a friend who has some insight into the banking system) is that the government has prohibited charging for UPI transactions. So every bank needs to maintain the infrastructure to integrate with UPI but don't make money off it. This leads some to not treat it with priority (a good to have, not a necessity) or treat it like IMPS/NEFT/RTGS (which only work in a fixed time window).

Overall the reliability is still quite good and with increasing reliance of the masses on it, it will hopefully get better.

[vinay_ys]

UPI usage/traffic has grown orders of magnitude each year. That kind of growth isn't handled reliably by the best of the startups even today (remember twitter in its early years?). Banks do invest a lot of money/resources into running/managing UPI but that alone isn't enough. There is no substitute for time and experience (talent) that's needed for the culture change within bank IT teams to mature, and their leadership teams – they have to shift their thinking to deal with the pace of growth and frankly the pace of changes as well.