Thank you for your answer.
I compared the configuration (lora-query -x config) of the two gateways. It is exactly the same (except the gateway eui). The devices that I use for now are an Adeunis network tester (https://www.adeunis.com/en/produit/ftd-network-tester/) and an ELA N’Track (https://elainnovation.com/lora-n-track.html).
Do you think the problem could come from these devices doing the join request only once ?
Regards