Some Amazon subscribe and save items not available after a few months

Many of the items in the Amazon subscribe and save items are not available after ordering them twice or thrice
For example Typhoo Green tea was the cheapest green tea available in the market
Now it is not available, and it is also not available locally in goa
Similarly there are other items, mostly consumer products like oil which are not available after some time.
It would be interesting to find out how items are included to subscribe and save instead of regular listing.