1. network problem: the game is unstable on the network, which may cause the gym to be unable to replace heavy objects.
2. Incompatible version of the game: If you play the old version of the game, it may be incompatible with the new version of the gym, resulting in the inability to replace heavy objects.
3. Game settings: In the game settings, there may be no option to turn on the replacement of heavy objects, or the button to replace heavy objects is hidden or disabled.
4. Hardware problem: If the performance of your computer or mobile phone equipment is insufficient, it may cause the gym to be unable to replace heavy objects.
If you encounter this problem, I suggest you check whether the network connection is stable first, and then try to update the game to the latest version to check whether the game settings are correct. If the problem still exists, it may be a hardware problem and needs further inspection.