What causes the charging failure?

What are other possibilities why my tablet won't charge?

There used to be a lightning bolt that appeared when I put the charger on but won't charge.

Battery replaced. it Worked for a week and then died.

I thought I bought a bad battery so I got another.

Battery replaced and this time it only worked for 2 days. It stop charging after that.

Brought it to a repair shop. Technician’s finding was neither the battery or charging port is the cause of charging failure. He stated that it was something else.

The tablet will work on a full battery, it just won't charge. There are no signs that a charger is connected but According to the technician who looked at my tablet, it isn't the port and it isn't the battery that is causing the problem but “something else".

Can somebody help?