Wall charging doesn't work, portable charging works?

Hi everyone,

I have a Samsung Galaxy Tab S2 9.7 WiFi running Android 6. 1 (marshmallow). Until a few weeks or so, I wasn't experiencing any issues wall charging the device. Then, I suddenly wasn't able to wall charge the device. I tried using various combinations of wall chargers and charging cables to no avail. Out of sheer desperation, I bought and used a portable charger. Then, bam! No more charging issues.

My question is this: Why am I able to use a portable charger satisfactorily, but not a wall charger? It doesn't make sense to me.

Thanks in advance.