A lot of players have good forehands in practice, either looping or hitting. But once a game starts, they stand facing the table, which is a backhand stance, and while their backhands are fine, when the ball goes to their forehand they mostly face the table, bringing their arm back to stroke rather than turning sideways. This leaves them with an awkward forehand stroke. They have only a small hitting zone, and this small hitting zone is jammed over the table. They have no time or space to actually do a normal forehand swing, and so end up blocking or doing awkward strokes. Their stroke in almost entirely with the arm.
Afterwards, they go to the table and spend hours practicing their already-good forehand, never understanding why they are unable to use it in a game situation.
When you serve, do you just serve? Or do you stop and visualize the serve first? And when you visualize the serve, do you visualize all of it, or just part of it? You should visualize the entire journey the serve takes, all nine feet of it (or 10.3 feet, if you serve crosscourt).
This video from ITTF International Table Tennis Federation shows an ITTF official performing and explaining step-by-step a real-life racket testing for an international competition. He checks for ITTF rubber approval then does the four tests for Flatness, Thickness, VOC gas, and Gloss. 