capacity in production order

Dear Gurus,

if i am creating production order for 100 qty,system calculate date from 17/03/2011 t0 20/03/2011 as per capacity,now if we chnge date from 17/03/2011 to 18/03/2011(instead of 20/03/2011)if we check capacity avialibilty system is showing following mess:

Capacity availability: Capacity available at all work centers

wherm should we can make setting system should show mess capacity is missing.

siva