When exporting a poster certain items from my library turn opaque.

I've had this issue for a couple of months now where certain items from my Libraries are turning opaque once they are exported. They either disappear altogether or show at about 70% Opacity. I've tried removing the "buggy" library items and re-adding them but that hasn't fixed the problem. I've tried rasterising or/and expanding the image but that doesn't fix it. My silly workaround at the moment is to draw a line over it and then Pathfinder them together..... 

Sometimes the library item works properly too.... I'm stumped