On the neck label of my Jack Daniels bottle I have the glossy node plugged in but as can be seen in the image -- it's not reflecting.
I tried adding another glossy to 'double' the glossiness but that didn't work either.
How can I make it glossy?
Essentially what's happening is the transparency is being turned into black which will just absorb all light. So it won't reflect.
How you should go about doing this, is make a material for your decal, should be a mix of diffuse and glossy, then, using the alpha from the texture as the factor, mix that with the material for the rest of the cap.
There's just a quick example I put together. The transparent parts of the texture use "Main Material", and the opaque parts use "Decal".