I would argue Booster and Beetle did break out. Maybe not in their original form, but they are solid B-List heroes.
Others:
New Gods
How many titles do New Gods have to get before DC realizes no one cares about them and they can't carry a series?!?!? The quality of several series has been phenomenal, but always cancelled in like a year. The only one people care about is Darkseid.
(I would also argue that being a supporting character in a mid-selling book really isn't being given a "chance"). Being put on the Justice League or in a movie or headlining a title is given a chance. A lot of the characters here I would say aren't necessarily in limbo, but haven't been given a huge push either.