There are two likely explanations to this issue. Firstly, if your previous hair colour (prior to application) was darker than chocolate (which is a 5.0 shade), you will simply not see a great deal of difference from using this product. However, you will notice the hair is shinier. This shine factor is a big indicator you have applied the product to a darker base, as whilst the depth cannot display (as the starting base was darker than the colourant base), the pigment molecules are reflecting in the hair and evoking the shine.
The second instance where you may see no difference after using the product, is if barriers are on the hair. Colour Restore shades cannot penetrate through styling residue, product build-up and silicone barriers. Therefore, you must clarify the hair before use. If the base shade is lighter that Chocolate, you will always see a result from the product as Chocolate is a very intense shade.