There are several bugs atm that can cause staff to glitch and stand around doing nothing or spinning in place. If you were unable to complete the task in talking to the cook, he may have been glitched on a task. If you open the staff panel, you can see what he is trying to do at the moment. Figuring out whats glitching them can sometimes help in getting them unstuck, as dragging them might only have them re-attempt the same glitched task. For example, my cook glitched on changing sheets. It wasn't until I canceled the sheets job at the source, did he relax.

As for undeliverable food. It could have been a bug, or it could have been that the guest got up and left impatiently. At which point, the food should give you the option to cleanup the undeliverable food. I've had times where I had half-cooked food on the stove.. and my cook had to pee, so he left, came back and the guest gave-up. But I couldn't get the cook to clean the stove of the unusuable food. But when the next guest arrived with an, order this did not block me from cooking. It just left an annoying cleaning icon on the stove that i couldnt do anything about.