It is important that your Hexoskin compatible shirt be fitted properly.
If the shirt slips on your skin, this generates noise on the electrodes which lead to erroneous heart rate detection.
If you place your hands on the two electrodes below your breasts and press gently without moving, you should see a stable heart rate measurement.
Consider using the strap that was included with the shirt on your torso to make sure the shirt always stays against your skin.
If this still doesn't work, maybe your device is defective.
Please read the following topic:
I'm getting a lot of inaccurate readings, is my device defective?