This is usually the result of a connection issue. Sometimes even the smallest bit of debris can cause a bad connection which stops the pen from working at it's full potential.
To start, we recommend that you try cleaning the gold connection pins between the battery and atomizer to alleviate any connection issues that may be occurring between the two.
This can be done with the wipe of a cotton swab dipped in ISO.
You should also make sure the connection between the battery and charger is just as clean so that the battery can fully charge.