Ann,
I have reviewed all the questions and answers from other experts.
Based on my experience of PS =52, An availability check is performed on the basis of the planned independent requirements. What that means Sale order ATP is performed against the dates in the planned orders that are created against PIR. The planned order dates would move if PIR dates are moved (normal MRP concept).
As I understand, in your case, there is a special situation due to a production problem and hence production cannot produce to plan order dates. In such cases it is the job of the Planner to manualy postpone the dates on the planned orders to a future best case date. When he does that system automatically inserts a firming indicator on those planned orders - Which means next nightly MRP run cannot change dates based on the original PIR dates.
Once the planner does this, run a new Sale order ATP in VA02 and the sale order would promise against New dates on the planned order.
Another way of handling this is to insert a planning time fence of say 15 days (depending on how long the production dept has a problem) and changing MRP type to P1 -P4 (cannot remember which one is best suitable for your situation) from PD. In this case, MRP would push the planned order dates beyond the planning time fence. Then Re-run sale order ATP and you shoud get new promise dates.
Please implement one of these solutions to resolve your special production situation (problem). Hope this helps you.
Regards,
Ram