It's usually caused by high temperatures, Fran.
Heat. I have that problem too.
I live in zone 9 where it gets blazing hot in summer. I get my tomatoes out as early as possible, and the harvest by the end of June is perfect, but anything after that the skins get thicker due to 100°+ temps. Then they go dormant until fall and start producing again. They will not set fruit if the daytime temps are 90°or higher.
Tomato skins get thick and tough when the heat is especially high. This helps the plant from getting sun burned. It is a natural reaction the tomato has for its own fruit. If they are in planter pots maybe you can move them not to get so much direct sunlight. If you have them in the ground, you might consider moving the area where they are planted next year if you can where they might have partial shade in the afternoon. Hope this helps, Fran.
Hi Fran: Here are some things you can do for next years crop to prevent skins that grow thicker than normal: water well while the plants are fruiting. A plant that “senses” a dry season will develop a thick, tough skin to protect the seeds. You don’t want to overwater, though, as this will dilute the flavor. Also, make sure the plants are being fertilized properly, so their growth is not checked by lack of nutrients. You can put a netting over the tops of the tomato plants to give some dappled sun during the period of searing sun and heat of the day. Good luck