One of the most common reason players have trouble blocking against heavy topspin is because they hold the ping pong racket too high. This would seem to make it easier to keep the ball down, but what really happens is that players end up holding their racket at different heights for different blocks, and so cannot really ingrain the proper racket angles. Instead, make sure to hold the racket pretty low, and take the ball off the bounce. This will allow a player to get a feel for the proper racket angle against a heavy topspin loop. It will also make sure you take the ball quickly, which not only makes the shot more effective, but catches the ball before the ball can jump because of the topspin, and so increases consistency.