Why battery percentage decreases when I connect my phone to charger?

i have a Blu Win Jr Lte phone. for the last one month when I am connecting my phone to charger the battery charge percentage is showing to be decreasing and the phone turns off. after switching on the phone, the battery charge percentage is seen to have increased and again starts declining. this goes on and on. I have updated my phone from windows 8.1 to windows 10 but the problem still persists.

I have contacted Blu support but I am getting reply as server unavailable.

I am from Kolkata. I have also visited the service center of Blu here but they told me that they are not servicing any Blu phones now.

could you please give any solution to this problem? and also please any way of contacting Blu.