I can't really answer WHY this happens, but I have had the same problem, but I don't really know where the problem is.
When I use the emulator, it usually connects fine but every now and then (seems like after several "runs") it simply fails to connect. It seems like the easiest way to get it working again is to close the emulator then restart it from the AVD manager.
Likewise, when using the B4A bridge app, I see the same thing. It will work great for a while, then eventually it fails to connect. Quite often when this happens I find the B4A bridge app to be locked up. It is difficult to exit the app and it is difficult to Force Close it.
That has been my solution when the B4A bridge stops bridging B4A to the device: I Force Close it on the device (via Manage Applications). It usually takes two or three tries, but eventually it will stop the process. After that I simply restart the B4A bridge app then restart/reconnect the bridge.
Hope this helps!