I'm not expert but looking it up shows that a sticker dieline is the cut line the die will use. On the graphic with your sticker art draw a big arrow to the cut line and print in bold "This is the cut line".
Make the line bold and obvious.
This site below says the dieline should be 1/8th" thick to allow for cutting error.
Take your graphic with the sticker art, cut line and bleed line (not the arrow and words), save it as an .eps (that makes it vector) and send it to them.
Writing this made me think maybe they need the actual line to be definitely in vector. If your art was ever saved as a .jpg or .bmp or other raster file types than the line may not be vector. With your illustrator file open draw a new cut line with the pen or line tool, save as an .eps and send.
If they still won't accept it then ask them what they are looking for specifically, with examples and possibly a template.