You can compare a working posting explorer to your AP posting explorer by pressing F11 (Setup Columns) note any differences. Adjust if needed.
I’m thinking the column headings are there but are not visible because you have overlapped a row over the column headings. Go to the AP posting explorer and click on a ledger line (make sure this ledger line is at the top of the displayed posting explorer) and drag the top line divider down (click the mouse between the blue title bar (.posting.index/description in my case (it’s a deep blue colour) and the top of the line to drag down. The mouse cursor may show you and up and down cursor indicating a drag is available. If not try to drag it anyway. This is eqivalent to stretching a column except you are stretching the row down.
If all else fails you can right mouse click on the posting explorer title (/ACCOUNT/AP/ACCEXP – Postings Explorer in may case (light blue)) and click Window > Default Setup which will restore the default setup for that window only.
The mouse is a powerful tool (good and bad).