My Tab A shows the red "X" as well when it is powered on and connected via USB to my Windows PC, however it is still actually charging. Check the charge percentage and then check again later to confirm the percentage has increased.
Mine was still discharging. I didn't notice the red X until it had been plugged in for a half hour or so. When I checked it I noticed that the battery % had gone down and then I noticed the red X.
I've gotten the red X once before with a Galaxy Note 8.0 tablet, but that was while I was connected to the wall charger and turned out to be a faulty charging cable. Definitely not a cable issue now because I tried multiple cables with my phone and tablet and my phone connects/charges just fine while the tablet doesn't. Oh well, I've been wanting to buy one of those multi-port charging stands anyway. Now I have an excuse to do it.
Mine shows a red x when connected to a mac using the original USB cable, but it still charges. I'm not sure why yours doesn't charge or why mine does even with the x. Maybe it is USB power (1.0, 2.0, 3.0, 3.1)? Did you try both ports? One port is normally faster than the other, but not sure if the power is also different.
I'm guessing it is a power issue. The larger devices usually need more juice than what can be supplied via USB. I had the same situation where I plugged my Tab A 9.7 into my Mac display (connected to a Mac mini) and it wouldn't charge. Plugged it into the fast charging cube for my Note 4 and it started charging right away. Situation is similar with iPads too.