The gateway must adhere to the duty cycle for the frequency band. So if too many devices try to join at the same time (and are using a slow speed) the gateway does not have the airtime to send the join reply packet.
Two things to solve this:
- make sure to use SF7 whenever possible
- do not send at exactly the same time. Uplink should work, but downlink will be an issue.
BTW, Are you OTAA joining the network every half hour? That is not something you should do. A session should last days if not weeks/months not ‘minutes’.