I have 3 silkie chicks and I think silkies may just be too dim to figure this out. They are not keen on roosting at all! I call them chicks but not they are 11 weeks old and I have been 'lifting' them in each night for a month now.
Are they your only chickens or are they housed with other breeds/ages? If so, it may be a pecking order related problem. The pecking order comes into play heavily during roosting time. Silkies are often low on the pecking order because of their personalities and their odd appearance. Additionally, young birds are usually low in the pecking order. If the coop is small and roosting space is tight, competition for prime spots can be stiff and a lot of pecking and pushing around can take place. Is it possible that your silkies are staying out of the coop to avoid that? Offering alternative roost spots and at different heights might help. Then the lower-ranked birds will have a roosting options that don't put them near the other birds.