Am I having trouble finding studs because my walls are lath and plaster?
Look for wall outlets. They are usually mounted on a stud. Then start measuring in 16 inch increments to find other studs. As a backup, I would use a small finish nail to hunt for the stud in the area that you want to put the TV in. Some studfinders have a "hi" and "low" setting that will adjust how sensitive it is to things behind it. Sometimes for instance, its harder on external walls because there is insulation in them. Maybe go to Home Depot or Lowes and buy another brand of stud finder. If it doesn't work you can always return it. Another kind of stud finder has a magnetic sensor and detects nails which are in the studs. You could try that type.