Karen, I live in zone 6. Not sure if we got our tomato plants last year from the same supplier? But the plants I bought were losers! The ones I grew from seed did very well. Last year there was a late freeze that may have harmed or stunted the young plants, even at the store before being sold, if they weren't covered well. The repeated cold nights don't help.

Years past I've planted some early tomatoes, like in April and May just in case the spring is fabulous. Then the ones I'd plant in June would out-perform the earlier planted ones. They are definitely a warm/hot weather plant, and cooler temps can be hard on them.

I use Miracle Grow...they love acidity. Keep them staked or growing up-right. Be sure to add OM (organic matter) to your soil. Oh, and when you plant it into the ground you can buy 70% of the plant, leaving only the top 30% above the soil line. That well help them get established well.

Possibly plant a variety of tomatoes from different suppliers, and then some from seed. Maybe one type will guarantee you a harvest. Happy Gardening!
Joanne