Sounds like the one with the dark (black) ring has died. Are they in an incubator or under a hen? If in an incubator the following information might be useful.

Humidity consistency is very important. I've always mist the eggs after turning them starting about the 8Th day. According to my notes the humidity was 85% & Temp 99.5F from day 1 to day 25 there after increase humidity to 95% and drop temp to 98.5F for the last 3 days. Turn the eggs 3 x day first week, then 5 x day second week, then 7 x day to day 25-finish the last 3 days without turning.
When you candle the eggs at 28 days, you should be able to see 3/5Th dark 2/5 should appear light. The dark side should have an irregular shape and the bill should be pointing up into the lighter side. You should be able to see movement and hear peeping.

Google "hatching Duck Eggs". Check out the sites that are sponsored by an University or State. Many of the hatcheries have good info also.

They are in an incubator, I picked up one of them today, just a little while ago. it was peeping, do you know how long it should be before it tries to hatch? I messed up my dates I think, and I want to be there, so I don't miss it

They should be hatching within 2 days of hearing the peeping. Is your humidity at 95%? If not, mist them every 2 hours until the humidity is up to 95%. Low humidity will harden the shells and not allow the duckling to hatch.

Another thing is don't open and close the incubator unless to mist or fill water reservoirs the last three days. The fewer openings keeps the temp and especially humidity constant during the most critical time.

I have a self contained system and never open it during the last three days of incubation and for two days after the hatching, you will have stronger and healthier babies this way. I modified the incubator and put glass windows in the door and both side of it to have a good view of what is going on.

What kind of thermometer/hydrometer do you have? There are very accurate combination one that are reasonably priced that will insure proper temps and humidity readings.

Also, keep an incubator diary (separate one for different species) recording the set date/expected hatch date/success and failures of each egg set. Each day record the temp/humidity three different and consistent times. Record openings of incubator and results of candling (candle at 10-12days, 20-22 days). Compare current readings with past hatches info. Try to mimic the successful hatch data to repeat the results. As you collect more and more information you see what is successful and what is not.

Enjoy and take lots of pics.

Lucky you!