Oddly enough, I created other similar ComboBoxes in other forms (using the same query.
I can only guess that you mean they work elsewhere.
If you're sure the design is correct (e.g. the column you expect to see is not set to 0" width or the column/field counts don't match) then post the sql for the combo row source. There could be an issue with your table/field names, such as "tbl-Main". What is tbl minus Main? Access doesn't know either.
Is the query against a local table or is it against a linked spreadsheet or csv file?
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.