1. you pot size probally to small beefsteaks need at least a pot size of a half meter or greater.
2. you soil is not in right ballance
3. you are watering irregularly
4. gardener delight is not tomato it is just an excuse that bad tomato grower use to claim they are growing tomatoes
5. tomato plants in pot need to be fertized more often then those grown in ground.
6. the temp of the water used to water the plant should at least 24 c.
I would continue but I don't want to embarrass you a more then I all ready have.