Thank you Mads for your reply,
reading the 802.15.4 standard I found that the ackWaitDuration must be 54 symbols for channels from 11 to 26 and not 12 like you said.
If I use a sniffer I can see the coordinator send 4 packet with a timeout of 2-3ms, this timeout in most cases is not enough for receive the ack from the end device.
This is normal for MAC/PHY library?
best regards,
filippo