During new detail line entry, some stock item(s) not showing as available to add to the Sales Order.
The most likely reason for this problem is not having a Base Price set against the stock item.
On the Sales Order, both the item pop-up look-up (accessed via the '...' button) and the Speed Entry automatically filter items with a $0 price. This helps restrict the look-up to those items that are typically sold, because a price indicates selling intention. If this item is regularly sold, then DBA's recommendation is to add a price for that item via the (Sales > Pricing & Discounts > Base Prices) screen, even if the price changes per customer.
If this item being sold is an exception occurrence then it can be accessed by clicking on the "Advanced" button just to the right of the red circular arrow on the item look-up (or Speed Entry). After opening the advanced filter, a check mark can be placed in the "NoSale" box to the far right of the pop-up screen to see the item.