If your shares, purchased before the ex-date, aren't eligible for corporate action benefits, then it's likely due to the following reason. Firstly, if the shares haven't been credited to your demat account by the record date, they won't qualify for corporate action benefits. However, shares bought on or before the day preceding the ex-date usually gets credited on or before record date, except under specific circumstances.

The reason could be a short delivery, where the seller fails to deliver the shares, leading to a classification as a short delivery and the shares are not credit to your demat account on or before the record date.  In such cases generally the exchanges will compensate the buyer following close-out process instead of auction.