Answer: Yellowing of lower (which are older) leaves may be due to a nitrogen deficiency. On tomatoes, however, it often is due to a fungal infection (usually early blight). You didn't mention any spots and indicated there was plenty of moisture, so we will assume the problem is nutritional or due to soggy soil.
I'd resume the liquid feeding once a week. If drainage is poor you might try to direct excess water away from the plants. Raised beds are a good idea as they help eliminate future drainage problems.
If the problem does not go away (and leaf spots still are not involved) you might want to dig up a plant and examine the roots for swollen knots (nematodes). Planting resistant varieties is the best way to deal with nematode infested soil.
Q&A Library Searching Tips