Hello, why I need re-transmit lot of time in API=1 mode from End device to Coordinator?

I’m testing two XBee Pro S3B 900HP, configured one of them as Coordinator and another one as End device. The end device is driven by a microcontroller; the Coordinator is connected to a PC by XCTU soft. The both modules are configured API=1. All is OK, except I need to send the API frame from the End device a lot of times (I did a loop in the microcontroller program) before the Coordinator could receive the packet. The delay is around 10 or more seconds. After the first transmission is received, there isn’t a delay for the next. Can you tell me what parameter, etc. I forgot in the configuration.? Thanks in advance.

Are you using CTS flow control? Have you verified the path using a Node Discover? Is the end device sleeping? If so, what sleep settings are you using?