You knew this was coming. In other words, someone wanted them to get a lot of attention, but not necessarily explode.
Via NBC:
Some of the 10 suspected mail bombs addressed to high-profile Democrats and others over the last few days were flawed and not capable of exploding, while others have yet to be fully analyzed, several investigators said Thursday.
In some cases, the flaws were substantial. In others, more subtle, they said.
The news comes after investigators said the devices appeared poorly made and that it was unclear if they were hoaxes or simply cases of bad construction. Earlier in the investigation, officials in multiple states had described the items as live explosives and a number of senior bomb techs briefed on the probe said they had all the components necessary for successful explosions.