Here are a few suggestions!
1. As Blue girl says, get him on the lead before he gets to the areas you know he's going deaf at..
2. Go back to basics and stick a long line on him in these areas so he has to respond with your
first call (if he doesn't you jump on the line and pull him back anyway) and give him a REALLY high value treat - roast chicken with garlic and lemon get my two without fail

3. Do it for 2-3 + weeks at least so you can get a good established behaviour pattern going - the more he learns to choose you over the picnic tables then more likely he'll be to do it!
4. If you don't use a whistle, you could try training him with that (whilst using the long line and roast chicken

) as the sharp tone often works more effectively than a voice command when they're distracted - Dave can be really slow with voice commands but usually bang on with a whistle. I also find with both of mine that their natural 'range' (i.e. how far they can be away from me and still respond well) is lot bigger with a whistle so I don't have to worry as much.
Hope that helps, it's SO frustrating isn't it!!!!!