Why when I drag my fields into form design, would only one field automatically come out as a checkbox, whilst other yes/no defined fields not? then when I try to change by right-click, the checkbox options are 'greyed out'. Any help greatly appreciated.

Open the table in design view and look to see how the fields are defined. They should be set to yes/no datatype and to have them automatically show on forms as checkboxes, the DisplayControl property on the LookUp tab should be set to checkbox. Even if the DisplayControl property is set to textbox in the table definition, you should still be able to override this on the form provided the datatype of the field is yes/no.

Tks, I tried that and also ensured Lookup tab selected for checkbox, but still can't get it. Strange too is fact that only checkbox created correctly is from an inner joined table's field. I suspect Access corrupted but ran repair/maintenance also. Any other ideas?