An item appears when it is below its minimum and there is not enough already on order to cover it. Here is how to check each part.
It is below its minimum
Compare the Available column against Min. If Available is lower, that is your answer.
Remember Available is not the same as On Hand — stock already promised to a customer order does not count as available to sell again.
Nothing is on the way
Check the Expected column. If it is 0, nothing is inbound.
If you did raise a purchase order, check it is Confirmed and not still a Draft. A draft order does not count as expected stock, so the item keeps appearing as if you had done nothing.
This is the most common cause by far.
Its minimum and maximum are both zero
Zero means "hold no stock". If you have sold more than you had, the item goes negative and appears on the report so you can buy enough to get back to zero.
That is intended. If you actually do want to keep some on the shelf, set a real minimum — see Set a minimum and maximum for an item.
It is in a different warehouse than you expected
Reorder points are set per warehouse, and the report shows one warehouse at a time. An item can be fine in one and short in another. Check the Warehouse filter at the top.
Still not making sense?
Gather this before contacting support:
the item's SKU code
the warehouse
what the Available, Expected, Min and Max columns show
any purchase order numbers you think should be covering it