How do I get my well-behaved dog back?

I have a 14-year-old Yorkshire Terrier whom I have had since she was six weeks old.
We went to obedience training for two years and she passed all her tests first time. For the whole of her life she has been very obedient and well-behaved and a joy to have.
For some reason she has now become very disobedient and just ignores me when I ask her to do things. She is a bit deaf so I always give her clear instructions and also hand signals. I have started practising some of the basic obedience training and she is quite happy to do that.
Any good behaviour is rewarded and this is really easy as she is very much food orientated. I have been giving her higher value treats at present.
She has a mild heart problem but this predates the change in behaviour by many years. She is seen regularly by a vet who is happy with her health, so I do not think that there is a medical reason for this change in behaviour.
Could you suggest a cause for this behaviour change and suggest what I can do to get back my lovely well-behaved little dog?

Jean Vale, by post

Changing training needs of older dogs

Dear Dogs Today
We enjoy reading Dogs Today in our household which includes two dogs - a Lurcher and a Border Collie. They have always had quite different training needs but now have a spectrum of geriatric special needs between them, too!
How can you adapt and if necessary re-train older dog as eyesight failure, deafness, senility and arthritis and changes in attention-span set in. It's taken us a while to adapt to how the communication between us the dogs has to change as their senses decline, and I'm sure other owners could also do with pointers and advice on how to adjust our behaviour to support them in their old age.
If we'd thought about it earlier and pre-empted their decline, we probably would have had more success training hand signals before their hearing went completely, for example!
Has anyone any hints?
Kat Joyce