I would suggest a soil test. You're probably missing or really low on one or more of the micros. PH could also be off.

I agree with everything taz suggested.

I also wonder about the quality of your seeds. I assume you have only been growing open-pollinated varieties and limited # of varieties? What are the odds of multiple-crossings over the years and your seed quality declining? Normally it wouldn't be a problem but it is something to consider especially if your other crops continue to perform well and only the tomatoes are in decline.

If the garden as a whole is falling in production then a soil test is even more important. Even organically it is possible for nutrients and micros to get out of kilter.

How about sun exposure in the garden? Has that changed markedly over the years - trees getting taller, new structures built, etc.? And of course there is always the weather to factor in. That one we can't control unfortunately.

Dave

If you have been planting those exactly the same spot every year you may need to relocate them to the different part of the garden.
I've read somewhere as an advice not to use same spot more than 3 years for tomatoes.