I use a dual-port 2-amp charger with an Amazon Basics USB cable. This works perfectly fine to charge my Nexus 4 and the Nexus reports "AC" not "USB" as charging mode. From the recharge time it's also evident that it gets plenty of juice.
However, when I connect my wife's Samsung Galaxy S Duos then it simply does not charge at all - the battery indicator does not get the lightning symbol, and the battery really never charges (even when nothing is connected to the charger's second USB port). When I connect the same phone to my computer's USB port, it charges just fine - even using the LG charging cable! - but slowly of course. The Samsung also charges well from any el-cheapo car charger (lighter socket) as well as from my expensive 3A car charger and from my LG AC charger.
What's the cause of this? How can a charger&cable combination work very well for one phone, yet not at all for another?