What are they planted in? (seed starting mix, potting mix, garden soil, etc)
Does this potting medium ^ also have fertilizer, or have you added any? What kind?
Are they sitting outside (sounds like it) or on a heat mat?
^ I asked the last question because tomatoes really, really want warm soil, not just warm air. You can by a simple, cheap soil thermometer for about 10 bucks. Ideal soil temp, not air temp, is 65-70°F.
Tomatoes originated in Central America and equatorial South America, and those are the conditions that make them thrive.
If your plants are outdoors in pots (or in the ground), this might be your problem. Black pots help; if they’ve hardened off (gotten used to full sun), maybe try to position them so that the outside of the pots can absorb more heat?
I know that when I first started growing tomatoes in Asheville (in pots, in fact), I planted in the second week of May, and they just sat there and looked at me until June, and then they took off.
You can also try a gentle fertilizer, like what I call “trout juice” (fish emulsion.) Great fertilizer for seedlings, but it stinks to high heaven!