How did they try to get Ralph out of the thicket?

They dislodged another rock.

As he did so, he heard Jack's voice from the top. "Heave! Heave! Heave!"
The red rock that he could see at the top of the cliff vanished like a curtain, and he could see figures and blue sky. A moment later the earth jolted, there was a rushing sound in the air, and the top of the thicket was cuffed as with a gigantic hand. The rock bounded on, thumping and smashing toward the beach, while a shower of broken twigs and leaves fell on him. Beyond the thicket, the tribe was cheering.