You just installed new rotors and the brakes still shudder or pulse under braking - frustrating, since new rotors were supposed to fix exactly that. In most cases the rotors are fine; something in the installation or break-in is causing runout. Here is what to check.
A dirty hub is the top cause
The most common reason new rotors vibrate is rust or debris left on the hub face where the rotor mounts. Even a thin layer makes the rotor sit slightly crooked, creating runout that feels like a warped rotor - see brake rotor runout. Always clean the hub to bare, smooth metal before mounting - see clean new rotors before install.
Uneven lug torque
Tightening the lug nuts unevenly or with an impact gun instead of a torque wrench can distort the new rotor slightly, causing thickness variation and pulsation - see what is brake judder. Torque the lugs to spec in a star pattern to seat the rotor flat.
Skipped or bad bedding
New rotors and pads must be bedded in with a series of moderate stops; skipping it, or parking with hot brakes right after, imprints uneven pad deposits that cause a pulse - see how to break in new rotors. This mimics a warped rotor but is really a deposit problem.
Ruling out the rotors
Genuine defective rotors are rare. Before blaming them, clean the hub, re-torque properly, and re-bed the brakes. If vibration persists after that, check for excessive runout with a dial indicator and inspect for a sticking caliper. Fit quality pads and hardware and bed them correctly.