I believe a lot of the disconnect is from expectations. Poly folks are likely not a majority. If they are, they're a very quiet major. (Not to imply that monks are torch-wielding villagers, but...) As such, most people you met will be mono and have mono programming to overcome. Many will have no desire to change their ways or their thinking. So a lot of your potential people just won't pan out.
To improve the math, try looking at your selection process. Random will be the lowest for success. Online sites and meetup groups will help because you'll be interacting with poly-friendly folk. For people you already know it may help to not be so shrouded in mystery.
If someone is developing an interest and you suddenly spring an existing relationship on them, it seems deceitful and easy throws people for a loop. The natural response according to mono programming is to back away. Instead, consider being open about having one or more relationships. Your target is less likely to flee because they know about stuff from the start. Build a normal, friendly relationship.
In the meantime, as a self-identifying poly, provide insight and perspective in discussions with that friend. Doing so will help them realize 1) you're interesting and concerned about their situations and 2) that poly thinking is reasonable and healthy. Once you have a friend who also understands how poly works and that it is a good thing and not whatever their programming said, then you can introduce the idea of them exploring with you. They'll feel more like they know what they're getting into rather than something that surprises them.