Hi Madhu,
There might be 2 chances for the issue:
1. when you are in creation mode of the activity check the configuration loaded is correct or not by pressing F2 in overview page. You have to add the attachment assignment block in activity bsp component using component usage.
2. You are saying attachment assignment block is visible in case of display or edit but not only on creation mode, check authorization restriction any thing is there by taking authorization trace in st01.
Mostly problem with configuration loading upon creation might be different than configuration loaded upon edit or display.
Regards,
Dharmakasi.