Tomatoes not producing seeds

[llsows]

I have been growing heirlooms exclusively for only 3 years and love saving the seeds. There are about 75 varieties in the garden planted 3-4' apart. Last year Amish Paste did not produce any seeds (but plants I gave to my neighbor did) and this year it is Violet Jasper. The plants are strong, huge and productive. I searched the web for advice and have yet to find anything helpful. Does anyone have any thoughts?