As others have said, it's difficult to determine the source of the problem without more information.
Typically a player who often finishes in the money, but rarely wins a big prize, is a player who plays tight throughout the tournament. To get a big stack, you usually have to take big risks. You have to learn when it is smart to take a big risk.
There is no easy way to learn this, since so many different situations are possible. Whole books are written about how to play at various stages of tournaments, and what kinds of risks are smart to take.
Another way a player might often finish in the money, but rarely win a big prize, is if they turn into a wild person after the bubble, taking risks that are foolish. This is less common, but I have seen it.
Also remember that even the best players do not usually make the final table.
If 150 players are paid, only 9 of them, 6%, will be at the final table.
If you are getting to the final table even 7% of the time that you are in the money in that tournament, then you are doing better than average.
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