700 r4 not shifting into 3rd sometimes

have a problem with my 700r4. when I first start to drive my truck the truck shifts through all the gears perfectly fine after a few miles (about 8mi)once I get to about 1,500 rpm to a dead stop (like a stop light or whatever)the truck will not shift into 3rd and will just rev high.does anyone know what could be the problem? Thank you