The plunger housing itself is probably out of alignment and may need adjusting so that plunger rod itself is aligned and sitting dead centre where the ball sits in the shooter lane.
You can adjust it by lifting up the playfield and using a screwdriver, loosening the screws holding the plunger housing in place inside the cabinet and then re-tightening them. It will be a bit of trial and error of raising and lowering the playfield to do a visual check until correct.
Also the plunger spring itself may be the wrong type (not enough tension) and that may be causing the ball not to launch with enough energy to make it all the way around the loop.
The manual says the spring is a blue 10-148-3, but you could go for a red higher tension spring.
http://www.rtbb.com.au/catalog/advanced_search_result.php?keywords=spring&osCsid=19b50d5fe5c9c4559d182f5fb5e95499&x=0&y=0Did the machine come with a user manual? You can download one for free here in PDF format under documentation:
http://www.ipdb.org/search.pl?any=black+rose&search=Search+Database&searchtype=quick
