Bad year for tomatoes

Hi folks I've grown tomatoes for many year's and this year has been a crazy one weather wise, the great weather during spring and the dull colder day's of early summer has my tomatoes in turmoil , I have average tomatoes on my first 2 trusses then nothing flowers but no sign of tomatoes  so ur ideas guy's will I use potash or tomato feed I have tried Miracle grow and phostrogen but not alot happening I know the weather has alot to do with it warm then a drop of 20 degrees , did I mention I have about 800 plants over 4 large polytunnels  my neighbours love me this time of year,I have Ferline and Crimson crush mostly and a few new ones many thanks folks.