I would like to ask a couple of questions;
1) Do you grow indoors/outdoors?
2) If indoors do you grow them in the same bed each year?
Where I am going with this I get the feeling the problem is in your soil!
As I was always taught never plant tomatoes in the same ground two years running.
If stuck for space,that is, its the only space you have then dig out the soil after cropping and replace it.
I am quite lucky in so far as I have three greenhouses so I can use a three year rotation which seems to work.
Occasionally if moving a compost heap I will replace the soil in a bed/s but that is 'hoss' work so I keep that to a minimum, more so now that I am knocking on a bit 8)