A few relatively recent misses for me, all since last summer -
Zorilla - Hoo Farm. Made many attempts to see these and was sorely disappointed. Might have seen a tail, but not enough to really count.
Chacoan Mara - Exmoor. One of several new species, and unfortunately one that had left the collection extremely recently. Still yet to see one.
Indian Muntjac - Chester. One had passed away, the remaining one stayed hidden over the course of my visit.
Black-Backed Jackal - Hamerton. The enclosure isn't viewable from up close and during the summer it's covered in long grass. No-one else saw them on that day either, and there were a lot of enthusiasts there.
Indo-Chinese Binturong - Exmoor. The only other lifer missed on this day, bit of a blow because that would have got me the set for Binturong subspecies in the UK.
Slender Loris - Birmingham. Have seen it before at Shaldon but very fleeting views. Visited Birmingham when they first went onshow, no joy. Didn't get round to visiting before lockdown and on my post lockdown visit the Loris House was closed due to COVID precautions.