Some laptops don't have quite enough juice to power external USB mass storage drives over the USB connection. This might or might not explain consistencies. If your drive uses external power from an adapter, this won't be an issue.
You should also make sure you're not plugging on a USB hub.
Next time you plug it in and nothing happens, run the dmesg command, or use "dmesg | tail" (to see the end of the dmesg output), to see what the system reports. *Something* is happening, just not what you expect.