Could you explain the problem a little further?
How intricate are these stencils and how tight do they have to hold for you to brush or spray through them?
Something like repositionable 3M spray mount is not working?
I grew up with this stuff in the graphic arts industry and have the fried brain cells to prove it. Try a quart of one coat (not regular) rubber cement and get a pint of thinner for it at the same time. If art stores even still carry it, they will sell cute little jars with glue brushes inside too.
Anyhow, mix the cement with the thinner to the least offensive thickness you think might work and apply to the back of your stencil. Let dry completely and you should have one sticky stencil as tacky as you determined it should be. If it does not stick, recoat. Make it totally high tack! The rubber cement thinner will not hurt the paint if you let it dry and cure to be safe. Use it to get any unlikely pieces of rubber cement off the walls though.
Not to offend you, but if a stencil is not sticking to the wall, are you confident your your paint will? Can your prime this situation first?