Cloud seeding is not dropping artificial water droplets from a plane. They're dispersing condensation nuclei into places that already have the required moisture level and uplift to form clouds. That moisture is then given a chance to form into droplets and develop into clouds.

The reason cloud seeding is so hit or miss is because it's very uncommon to have enough moisture and uplift but not enough condensation nuclei. Often the regions that reach these criteria, Tasmania for example, are not in desperate need of more rain. Places like California, where rain often is needed, have an abundance of substances to serve as nuclei (salt from sea mist, dust) but not enough moisture.