As yamahauler said, obviously make sure the surface is spotless. NO oils, solvents of any kind. Sometimes a heat gun will accelerate the degree of adhesion, but be careful not to get the heat too close to the graphic or they can melt. Try applying the decals dry, if you have been trying the "wet" method. Use masking tape to mock-up where the print goes, then pull the liner off and start to squegee from one end out to the edge. If that doesnt work, in the vehicle graphics/wrap business there is such a thing as a pen that will seal the edges of graphics. Google seal it pen, or Fellers online catalog.
Heres a link to one I got in just a quick search:
http://richardsonsupply.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=SEAL&Category_Code=VS_MI
Sometimes, unfortunately vinyl will not adhere to some types of plastics. Just the other day a customer wanted some stickers for a gun case. No problem, right? Wrong! No matter what, these things would not stick to the plastic. You could also just inquire as to what kind of vinyl your vendor is printing to and if he possibly had a vinyl with a more aggressive tack to it that he could try.
Those are my ideas, good luck!