Ok sounds good. Have the card readers which are difficult to read at been moved, or other metal objects been moved closer to them?
Could also be the make / model? Are they all the same type of reader at these 3 places? A recent firmware update may have changed their performance? The new NFC v5 spec came out recently… terminals might be getting rolling upgrades…
I realise the new nfc spec is for longer range operation, however this could also backfire a bit for older transponders not compliant with 15… not sure… anything is possible, especially with badly implemented updates. Payment terminals are typically focused on active-active NFC with phones, and kind of eschew cards and fobs… so I would not be surprised if an update focused on that and basically blew off any detrminetal effects it might have on passive card communication.