A couple of things come to mind.  How much sun are your eggplants getting?  Are some more shaded? 
Are the eggplant in the picture, firm? 

If eggplant does not get a full day of sun it could cause the eggplant to remain green when it should be turning purple.  The second thing is if the fruit remains on the plant and starts going past maturity it can turn green.  Typical harvest time for eggplant runs around 70 to 85 days.

Was the fruit in the picture shaded from heavy foliage on the eggplant itself? 
I don't see yellow blotching or gray blotching which would indicate a disease of eggplant and peppers called gray wall.  This can be caused by cool, wet and cloudy conditions and/or compacted soils. 

Thanks for your business.
Linda

They tan green fruit on the left had been that way for a few weeks. It had stopped growing. And yes, it was fully shaded by the plant canopy and on the east by the shade of a tree and a shrub. The right eggplant had just gotten this big and yes, it was also in the shade of the plant leaves. I've harvested full sized eggplant from the plants in my garden that have double the left hand bulk and probably more. They were beautifully purple.

Thank you for letting me know that they were shaded.  Thanks for your reply.  Has it been hot and dry in your area lately? 

Thanks, again for your business.
Linda