Store bought flour tortillas have lost moisture over time and will become dry or stiff if improperly heated. To re-heat tortillas, they should be lightly steamed so no more moisture is lost.
Put a tortilla on a plate, covered with a damp paper towel. Alternate tortillas with paper towels. Microwave for 30 seconds to one minute, depending on how many tortillas.
Preheat the oven to 250 degrees. Wrap a stack of tortillas in a damp dishtowel and place in a casserole dish of similar size. Cover with a lid or a piece of aluminum foil tightly on the dish. Place in oven for 20 minutes.
Once the tortillas are heated, transfer them to a tortilla warmer to keep them nice and hot. Or on a plate under a warm damp dishtowel.
Also, the brand makes a difference. Bend the package of tortillas before you buy them, they should not be stiff. Compare the bend ability and the thickness to other brands. Mission makes a thicker tortilla and Guerrero makes a thinner one.