You need to cut all the dead flowers off to produce new ones. If the plants were that large when you bought them they are probably very root bound and water has a hard time soaking in.
Linda C

I also live in MA, as you see, and have grown lavender lantanas in hanging baskets for many years. They are most certainly not drought tolerant--whoever told you this was wrong. I water and fertilize every single day. 1/4 teaspoon miracle gro to one gallon of water. Water til the water is coming out of the bottom of the basket. You will get large plants packed with flowers all summer.

They need full sun here. And water every day--twice when it's in the 90s. And the fertilizer--or few flowers. It's a lot easier than it sounds and the results are gorgeous.

thank you for the advice. Should I trim them back at all to promote new growth?

I never trim these or deadhead them. They are self-cleaning. But if you have a lot of dead from the lack of water, then I'd clean that out to get a fresh start. Good luck!

The seed pods produced by lantana do indeed tend to deter more flowers. I dead head them occasionally not to make them prettier, but to encourage more flowering and especially fuller branching.