When asking for help on how to use MuseScore, it is better to use the support forums (Help / Ask for Help) rather than reporting your difficulty as a bug. Anyhow, in order to help you better, we would need you to attach the score you are having problems with and give precise steps to reproduce the problem.
I can guess, however, that you simply haven't incldued any dynmamic markings to tell MuseScore how loud you want it to be. MuseScore looks at the dynamic marking before and after the diminuendo to determine how it should work. If you have not placed a quieter dynamic after the diminuendo, then by default nothing will happen on playback. Yu can force MsueScore's playback to get quieter anyhow using the Inspector, but human musicians will still appreciate an actual dynamic marking.
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When asking for help on how to use MuseScore, it is better to use the support forums (Help / Ask for Help) rather than reporting your difficulty as a bug. Anyhow, in order to help you better, we would need you to attach the score you are having problems with and give precise steps to reproduce the problem.
I can guess, however, that you simply haven't incldued any dynmamic markings to tell MuseScore how loud you want it to be. MuseScore looks at the dynamic marking before and after the diminuendo to determine how it should work. If you have not placed a quieter dynamic after the diminuendo, then by default nothing will happen on playback. Yu can force MsueScore's playback to get quieter anyhow using the Inspector, but human musicians will still appreciate an actual dynamic marking.