I had this same problem with my Samsung S6 Lite tab. Tried every single ***** charger in the house with it! From samsung to unofficial ones absolutely nothing was working! But, I got a new one and the charging speed then went a little faster but not fully fast charging. But then, I got my new phone with a fast charging block and USB chord. So I honestly think it depends on the charging block cause after that, my tablet now does fast charge with my official samsung fast charger