Writing NFC tags

iOS and iPadOS

Hi all,
I'm unable to write NFC tags on an NTag213 chip with an iPhone Xr under iOS 14.4 and the NFC Tools app. Once I wrote my text, the app asks to have my iPhone close to the NFC chip. I made tens of attempts in vain.
Could somebody tell me what is missing? Is there anything to do before using an NFC chip for the first time? Is there anything to set in the app?
Many thanks in advance.