This can be caused by a number of problems.

Possible Problem 1: The gift was uploaded through the TeamRaiser gift upload with a receipt number greater than 0.  If you do this, the system assumes that you have generated a receipt offline and that the number included in the upload corresponds to that regenerated receipt. To fix this, delete the transaction and re-upload it.

Possible Problem 2: The tax deductible value is not greater than zero. If the value of goods and services are equal to the transaction, then there is no tax deductible value associated with the transaction. In simpler terms, the constituent bought an object (or server) at its proper value so no part of the transaction was really a donation. So if you have a mug worth $5 and someone donates $5 and you give them that mug, it was really a purchase and not a donation and thus no receipt is generated. If, however, you have a mug worth $5 and someone donates $100 and you give them that mug, it was a $5 purchase with a $95 donation and thus a receipt is generated. This cannot be resolved by an admin in the UI. You must contact support for assistance in this.