I bought this tablet recently in Tenerife.  Can find no information about the manufacturer.  

Now the battery will not charge through the mains adapter. It did charge at first.  However, I have found it does charge through USB connection to my PC.
When it charged previously a light at the connector would be red.  When it was not charging it would be green.  It is red while USB connected. 

Also it tells me now that it is charging, but if I remove the USB connection it stops charging. It has a screen showing battery condition in the form of a graph.  I could see the graph of battery % decreasing gradually.  A few days ago there was a sudden drop as if some App had used a lot of power.  

I decided to remove all Apps and did a factory reset.  This made no difference regarding the failure to charge. I have tried shutting down completely (hold the off button for a while and it asks if I want to power off)  

This made no difference.
Any advice please?

Edit #1: Thanks ce4 for your response. The charger is stated to be 5v and when I check it with a voltmeter, it shows 5.2v. Also I have tried pushing the plug into the tablet to varying amounts and makes no difference. Another point maybe I didn't make clear is that when I connect with USB only, the light does not come on but the device tells me it is charging.

But when I connect the power supply as well, the light comes on red as if indicating charging. But when I remove the USB leaving the power supply connected, the light goes back to green and is not charging from the power supply.

Despite apparently charging on USB, the percentage of battery left charged has not yet increased from 10%. I understand that USB cannot deliver high current. It appears you are right - something wrong with the charger - maybe it is not the correct one for this model - I guess it needs to be able to deliver power at a higher current level?


Question: Is potential energy and "work done" the same thing?
Answer: Potential energy is a concept of the energy results from the object position 
as a concept it refers to the work you have to do to move the object from $U=0$ 
To any another point 

at $U= 0$ we call this point the reference point and it's like how to choose the origin of the coordinate system