First thing i would check is to make sure the units are

1. Wired correctly. There should be a hot and a neutral and the interconnect wire. It's possible you have these reversed. When you test one detector, do the other's sound?

2. Power present? Is their a breaker tripped, or perhaps the bad connection is elsewhere in the circuit. Use a meter to check to see if the hot and neutral have 120V across them.

If all else fails, you can try replacing them all. There might be some incompatibility between the brands. I like the Kidde ones I have installed.

Combo Smoke and CO is a stupid idea by the way (though it may be required by your local code). You can't place a combination unit optimally. The CO goes in a different optimal placement than the smoke.

Thanks for the input - I'll try those suggestions.

Agreed on the CO combo - it was required. We have actually useful ones as plug-ins.

Our brand is also Kidde.

Thanks!

Also check to see if yours are on the recall list

http://www.cpsc.gov/en/Recalls/2014/Kidde-Recalls-Smoke-and-Combination-SmokeCO-Alarms/