First of all re-mount the wheel by setting the bike on level ground, then opening the QR tapping the wheel so it settles home, and closing the QR. This is the wheels natural position and if you get into the habit of always mounting the wheel on the floor it'll always return to the same place.

Now check for centering in the chain and seat stays. If it's off center, especially in the seat stays, it could be a mis-aligned frame, or dropouts not perfectly positioned, or a bent axle.

Begin the process of elimination by opening the QR and rotating the axle in 90 degree increments and testing all 4 positions. If the wheel doesn't move the axle is fine, and since you checked the wheel's dish it might be the frame. Before going crazy, be aware that many frames don't have symmetrical chainstays so it might be perfectly fine. Also, double check for dish by mounting the wheel backwards (cassette on left). The rim should be in the same place, otherwise it is the wheel after all.

To check the frame, make a small loop in a piece of string and slip it over the QR skewer. Loop the string around the seat tube, and bring it back to the the other side. Pull it tight and close the QR trapping it. Measure from the string to the rim (if the tire touches remove it).

Repeat the process with the string just above the BB, half way up the seat tube trapped over a bottle boss, and at the top trapped above the top tube. All three pairs of measurements must be equal right and left (though not equal to other pairs) within 1-2mm. If not, than the wheel isn't centered within the rear triangle, something that can be fixed by filing either dropout, but you need to know what you're doing.

Note this test only determines the wheels position in the rear triangle, and if this is off it might indicate other frame alignment issues, so you might want to have someone check everything.