Why aren't dynamics in Musescore affecting my playback after a midi import?

I am currently working on a score featuring a mash-up of different pieces and I was planning on using midi files to help speed up the process and all. Instruments will need to change volume using dynamics, as the instruments constantly change in importance and hence balance shifts are needed. However, when I try to add dynamics to the downloaded midi parts, the dynamics do not effect them in playback and they play just as they were downloaded, ruining the balance. Now, I know you can also use the mixer to adjust the volumes of instruments, but it only really effects the constant pitch as far as I can tell; it cannot change the pitch of instruments halfway through a piece. Additionally, I checked to see if the program just was not working correctly, but when i just typed in the notes needed, the dynamics did take effect in playback.
I would love it if someone knew what was going on, as I sure do not. I have tried everything to make the dynamics work for the midi piece. I have tried numerous tricks online that say how to make dynamics work. I even copy and pasted the notes into a different page so they technically would not be imported any more, but that did not work either. I would like to avoid having to type monstrous amounts of notes and rhythms for hours just so that the playback sounds right. Thanks for the help!