Hi,
check the availability rule set for your sales area
for your sales area, you can see three options such as
1. one time delivery
2. Delivery proposal
3. complete delivery
I believe you might have set ONE TIME DELIVERY option . change it to complete delivery or delivery proposal and try
Path: SPRO-->SALES AND DISTRIBUTION-->BASIC FUNCTIONS-->AVAILABILITY AND TRANSFER OF REQUIREMENTS-->AVAILABILITY CHECK-->AVAILABILITY CHECK WITH ATP LOGIC-->DEFINE DEFAULT SETTINGS
try and revert
regards,
santosh