This is a condition called, a failure to set.  It is fairly common and can last all season.  The following steps can be taken:

1. Be careful not to over fertilize.  Application of fertilizer containing nitrogen from ammonia can cause tomatoes to rot on the ends before they get very far or no tomoatores form at all.  Large nitrogen applcation will grow your tomatoe plants tall and lush, but can prevent the plant from taking up calcium, which is needed for the tomatoes to fruit.

2.  Maintain your soil pH.  Tomatoes thrive in a pH of 6.5.  Apply garden Lime, or Fast Acting Lime to the soil around tomatoes according to instructions.  You can also apply Gypsum, or Fast Acting Gypsum tot he soil around the tomatoes as a source of calcium. 

3.  Water enough.  Water your plants regularly is they are without rain.  Plants need about one inch of moisture each week from rain or irrigation.

4.  Mulch.  Mulching your tomatoe plants to help conserve moisture and to avoid soil temperature variance.