I had a problem with my tomatoes as my first 2 plants died and I think this is due to the clay soil that we have here - best way to help combat that clay soil is to create a mix of sand, soil and compost and work it through the soil, this will give drainage to the plants and also help to combat the clay, your plants will be a lot happier - I should have learnt this prior to planting my first tomatoes and then they wouldn't have died.
I have since made small beds for my tomatoes and peppers and they are going strong
General Gardening Spain