The trouble is that the plastic surface isn't smooth enough to accept the small and relatively inflexible decal.
Firstly wash the part, it may have release agent on it which will cause the decal not to adhere.
Then I would paint it with a coat of Future or Johnsons Kleer floor polish which will improve the surface no end. Get hold of a bottle of Micro-Sol and Micro-Set and paint the area you are going to stick the decal with the Micro-Sol, then stick the decal on. The Micro-Sol softens the decal and makes it more pliable, which will now also adhere to the shiny surface much better.
When dry paint on a bit of Micro-Set which will cause the decal to stretch slightly and any creases and blemishes will dissapear and, I kid you not the decal looks like it is painted on. You could go for the delux route and put a coat of Kleer over it to seal it and then a final, all over coat of a clear varnish in either Matt, semi or gloss but you will have to put a final, all over coat of something on it.
You will be amazed at the differrence.