As you lower the buffer size, this adds greater strain on your computer's CPU and it will eventually fail to pass audio. To improve the performance of your computer, you should work through the guides to optimise your computer for audio and remove and disable any other devices and services that may be adding a load to the CPU.
However, there are also issues on Windows 7 if the device is not connected to a USB root hub. This usually means that it is not possible to achieve buffer sizes of less than 3ms. This is particularly prevalent with Windows i-series processors (i3, i5, i7) and is explained fully in the answerbase article here.
Desktop users can add a PCI or PCIe USB card, which will work as long as the card does not have an internal, non-root hub.
Notebook/laptop users cannot add a USB 2 ExpressCard, as these cards attach a new hub to the existing USB controller and you still cannot connect directly to the root hub. In this case, a USB 3 ExpressCard can be added provided that you are using the latest drivers from our downloads page.
If you continue to experience problems, please contact Focusrite Technical Support.