pin sleep delay

I’m running 2 xbee series 1 modules with default settings other than SM which is set to either 1 or 2.
The first module is only used for xmitting and the second is only used for rxing.
When I first power up the rxer, my code wakes the xbee, then looks for rf activity. If there is no RF activity, the module should sleep (doze) until I deassert pin sleep. My code sleeps for 6 seconds and wakes for 1 second.
My problem is that on initial powerup, the xbee will not sleep for 15 seconds, then on subsequent loops the desired sleep period runs fine. Obviously 15 seconds at 50mA is not desirable, when I only need 1 second to check for RF activity.
Can anybody tell me why there is this 15 second delay at initial power up ? Others seem to have got round the issue by holding reset low instead of using sleep, but the sleeping current is then higher (5mA)than by using the normal sleep pin (0.18mA). I have now got a mixture of methods, so that the reset is held low at first power up, but after the first wake up command, the code reverts to using pin sleep. This has reduced the delay to 10 seconds but not removed it.
I have the modules set up to not associate and there is no coordinator. DH and DL are both 0.
Pulling my hair out here …