There was a thread a month or so ago about, I think, senior dog no going on marks. If I remember correctly it wasn't consistent sometimes perfect then the next day a no go. I'm not sure I have it right. I know there were different opinions about how to work through it. If I remember correctly the OP decided to put the dog up for a week or two for the dog to think about it and hopefully the dog would regain drive.
I searched but couldn't find the thread. I am just curious how it worked out for the OP.