There are several small things that can be the problom: 1) check that the battery us infact the 100% compadable with your player. 2) check for any corrosion of the positive and or negative sides of the battery.(you said they were new so thus is probably not the problom) What model of player do you have?
What type of charging port does your tablet use? Some times there can be gunk inside the lighning micro or usbc port. Gentey imsert a WOODEN toothpick and scoop out any stuff. What is the make and model of this tablet?