I am an exec ad rep for a huge property listing portal, having a hard time getting realtors interest to post their listings for free, any suggestions?

I know that realtors spend a lot of time and money on advertising but why am i having a hard time getting realtors to sign up absolutely free? Are you getting bombarded with such offers all the time or is it a time issue?? I can even post them for you and then you just have to activate them for free. I do have a lot of realtor friends that jumped at the chance but it seems i am not sure if others just haven't seen my invitation email or how to approach them. If you are a realtor, would you rather get a phone call or an email with a free offer to post all your photos and listings for free? I just don't want to spam anyone or dont want to waste any time and i guess my first opinion would be to call (?) Thank you for your reply...

Arick