Did you clarify the hair prior to application? With light blonde, tonal shades, this is one of the biggest reasons why the colour looks unchanged following application. The hair must be free from barriers prior to using Colour Restore Silver Blonde. Once clarified, the pigments within the shade will be able to enter the hair and evoke change.
Alternatively, because Silver Blonde is a metallic toner, you may have a base that is just too dark to display metallic tone. If you are looking to achieve a pure Silver Blonde result, you firstly need to make sure the hair is lightened to a platinum base, before application of Colour Restore Silver Blonde. Silver Blonde is a grey hued, light blonde toner, it is not an actual grey depth colourant. So, the result from Silver Blonde should be a metallic, greyed blonde that still retains its light depth.