Get the brightest flashlight you can and make sure where you candle them is really dark.
How far along are the eggs? If they are towards the end of incubation (day 14 or later) all you SHOULD see is dark except for the air sac. Once the chick starts filling up the egg, it looks all dark.
ETA: I had a hard time seeing into my welsummer eggs. I had to sit still for a while to distinguish movement in them. Even my maglight wasn't bright enough, so I bought a super flashlight for next time with 9 bulbs in it. Gosh that sucker is bright!!!! And it only cost me $5.