Why pin wake up sometimes doesn't work the way I expect?

Hi, I have the next configuration on sleep mode on Xbee XR 9000 RF:
SM 5
SP 64
ST 1F4
WH 64

When I have to transmit some data I wake the xbee with a pulse low and then I transmit data and works fine, because the pulse it’s so short the xbee wake up for ST time. But sometimes the xbee wake up and doesn’t transmit the data, and it lasts on 2 seconds, why is it happening that.

I would suggest logging into my.digi.com and submitting a case. When doing so, please make sure that you are providing all available data.