How about trying to start out on Post Partum or well baby nursery? Lots of folks break into L/D by entering into these areas, first. Just a thought.
Also, try to get NRP (neonatal resuscitation program) certification, to show them you are very serious about entering into OB nursing.
Networking helps, too. Have good friends/colleagues in OB who can speak on your behalf, to maybe help get you hired? Might be helpful. Make sure your references can really help you as well, extolling your virtues as a nurse, in general. Most OB managers DO appreciate prior experience in places like the ED, ICU/CCU or med-surg. Such nurses have proven they have wonderful time management skills and can handle high levels of acuity easily.
Also, you could become an AWHONN member (and put this on your resume). See www.awhonn.org
GOOD LUCK and KEEP TRYING! There surely is a manager out there smart enough to see your merits and hire you, sooner or later.