Shielding paint, when still wet, often has large resistance. The conductive particles are still floating around, separated. You have to have patience and only measure it when properly dried.
To make good contact with pointy probe tips, the conductive paint needs to be fairly thick. Make some thick blobs on the painted region, far separated, and wait for them to totally dry. They'll be the contact-spots for probes. Lay the tips on them at an angle, to give widest contact area.
That, or attach small pieces of conductive copper adhesive tape to the well-dried paint in two separate spots. The tape's conductive adhesive makes wide-area contact, even when the probe-tips could not.
Last resort: embed very clean, very thin copper wires in the wet paint. When dry, measure resistance between the wires. First tape them down so they don't shift at all, since they can easily crack loose while inside the paint, yet they'll appear to still be in good contact.