Some breeds are more prone to brood than others. Silkies and Games are most noted for their ability to brood. I use Sumatras and Phoenix since the Silkies were all killed.
An investment in a couple good Silkie setters is not too much hardship and they DOI work well.
I never set hatching eggs beneath a broody until she has set for a week, by then, they usually stay broody.
You cannot force a hen to brood, to help them stay interested, give them a dark, secluded spot to to the job. They can be moved after they are setting tight, at night. I do it routinly.

Yea thats what I thought. I have silkies and a plymouth barred rock that go broody often, but maybe i am catching them when they arent settled in enough like you said. Thanks:)