I can remember watching a retriever trial and on the last drive of the day, some birds had been shot and fallen in some fairly light cover. The air temperature was dropping extremely fast and a cold mist was beginning to fall.
You would have thought the scenting conditions would have been quite good, but I watched as two dogs failed to pick their retrieves. Both of them went straight over the top the birds. To this day I haven’t been able to work out what was going on, but I can only assume that the dropping temperature combined with the mist reduced the scenting conditions to nil and the dogs couldn’t see the birds.