Does it help to press 'Z' (instead of 'TAB') to highlight? That's worked for me so far..
('Z' is "reverse highlight". From the wiki: "Highlight highlights all characters and usable objects, showing their names and displaying any that are obscured by walls. Reverse Highlight simply highlights them in reverse order. If something is untargetable because it's behind another object (e.g. enemy behind a door or remains behind a desk), you can target it with reverse highlight.)