USB Plug N Play problem

We are using a USB slot for providing a plug’n’play interface for memory
stick devices. We initialise this feature via the command “devb-umass cam
pnp $”. The plug and play feature appears to work perfectly for approx 10
insertions, suddenly the automount feature appears to stop working. No
directory /dev/hdx appears anymore. Is there any known errors in the
current io-usb implementation or devb-umass driver?