There are default purchase items.
I guess that drop ship orders are the problem: I had 2 drop ship sales orders and a few normal orders for the same item. I could not create any Purchase Order for this item via Create purchase orders maintenance (both running from maintenances and from sales order via Create menu).

I had some other item which was used in normal orders only and I could create purchase orders, though, I guess there is a bug, because the location on the purchase order is blank while on the corresponding sales order ("back to back" order logic) is not empty and as result Stock Reservations records are not affected by these Purchase Orders. The reason for this is blank location on the Purchase Item. I'll report a bug.

Another problem with stock reservations is that reservations dissapear when we delete the purchase order (OK purchase order, then unOK the purchase order and delete it).