Certified GKH Gardening Expert
This, actually, has several causes. I would suspect, first, that nutrient levels could be off. Test the soil for deficiencies, or pH swings. These articles will help:
Should soil tests confirm deficiency, I would advise amending accordingly. These articles will help:
If this doesn't correct your issue, then you may be at odds with your environment. Temperature or humidity levels can halt fruit production.
This article will help:
And, finally- This article will give you all of the information that you will need to be proficient at growing tomatoes: