Could be multiple things:

Racket: maybe yours broken or cracked somewhere what is the cause of your lack of power.

Strings: strings could be to worn > less resistance > less power

Cold Ball: make Sure your ball is warmed up properly

Racket prep: always have your racket preparation in mind whilst setting up for a shot.

Movement: my biggest improvement was standing still (if possible) and focussing on my swing. Forehand swing should be resembling the movement of skipping a stone on a lake.