There may be instances in OS X where devices that require a third-party driver (such as input devices and cameras) will not properly load at bootup. While a device may load when attached to the system after bootup, it may not be recognized properly when attached at during startup. When this happens, it is usually a problem either with the system's boot caches, or a problem with the developer's configuration of the driver being used for the device.
One way developers create drivers in OS X is to start with a generic driver class (i.e., a "printer device," "storage device," or a "camera device") and then build off of it, modifying its components until the driver works with the specific features of the hardware at hand. With this setup, if there are problems with the developer's configuration of the driver then there may be problems with which driver the OS matches to the hardware at bootup. Third-party or more specific drivers will need to be given priority over a generic driver class, otherwise when booting the system may match generic drivers to the device.
This can usually be tested by unplugging the device and then plugging it back in once the system has loaded. Since a driver for a device will stay loaded until the device is removed, unplugging the device will spur the driver to unload and then select a new one when the device is reattached.
If the device loads properly by unplugging and plugging it back in, then it is likely that the driver itself is not set up to be preferentially selected for the device over the generic drivers. Your best bet is to look for a driver upgrade from the developers.
Another possibility for the driver not loading at boot is that the system's boot caches may not be properly configured to load the device. If the device is initially matched to a generic driver, then this driver may be loaded into the caches and will be continually matched to the device each time the system is booted. Granted, installing a third-party driver should change the Extensions folder's modification times and spur a rebuild of the boot caches, but it is possible the generic driver could still be matched to the device.
If the device is an external device, try clearing the boot and kernel caches using a maintenance utility like OnyX, MacCleanse, MacPilot, among numerous others, and then loading the system with the device unplugged. This should build the cache from scratch for all devices the system can detect, which should not include the device at hand. When the system is booted, plug in the device and ensure it loads and is working properly. After this, restart the system to test whether or not the driver gets properly associated with it at startup.