Big possibility that it's broken (Vortex) but in case it's not,
Set it to max magnification
Set parallax to infinity
Start with the diopter adjustment all the way in, and then briefly look through the scope at a white wall. If the reticle is not in focus, make 1/4 turn adjustments on the diopter and try again. Keep doing this (with like 15 second breaks between looking through the scope) until you have a crisp reticle and your eye doesn't have to adjust to see it clearly.
If you can't get it in focus, or it doesn't stay in focus through the full magnification range after doing this, congrats you get to experience that famous warranty the vortex fans always rave about.