In think there is something wrong in your experimental set up.

The sample might get poled in some direction and there is a phase shift occuring.

Please perform the experiment with a fresh sample.

Thanks for your quick response Sougata Koner Sir

We did it three times with different samples only but still it's showing the same pattern and also every time we observed different ME coefficient values

Then I think please verify the experimental setup.

The ME coefficient should trace same trend for a particular sample in a particular condition of experimental condition.