This problem has been observed on an HP iPAQ 2490 with Widcomm BT stack 1.7.1 build 1424 and an iPAQ 114 with Widcomm BT stack 1.8 build 5600.
The environment is such that the PDAs have recently been powered up out of standby (I don't think you can turn these models fully off) and a Bluetooth discovery has been started via DiscoverDevicesAsync(). The problem is that no device discovered notifications are received by my application.
I reckon that this is a long standing issue that I have not fully understood before. Previously, I thought this issue was caused by the underlying BT stack becoming unresponsive (indeed a separate issue), the blue LED is flashing on the device but you cannot connect or discover devices, even using the 'Bluetooth Manager' application that comes with the PDA. However, in the case of the above I was able to discover the BT devices in the vicinity using the PDA Bluetooth Manager, but could discover nothing with my application.
Soft-resetting the device allowed the discovery to work correctly once more. I would like to blame my application, but it doesn't do anything special other than start the discovery and receive the device discovered notifications. So could there be a problem in Bluetools with discovery?
I repeated the test on a Fujitsi Pocket Loox N560 with a Microsoft BT stack, brought out of standby after several days of inactivity. No problems with this one, devices were discovered fine.