My guinea pigs still aren't used to me?

About 2 weeks ago I adopted 2 male guinea pigs from the shelter and named them hickory and bean. The worker told they never have been handled and so I was ready to work with them. they let me touch them but if i stick my hand in the cage they run to the corner and run if i try to grab them. They come and except lettuce and veggies from my hand but wont let me grab them. I gently put my hand in their and taint them but nothing! Wat should I do????