@johnbachman I already had a response from the support team. It can be more than one reason, in my case the cause of this problem was that I was not putting all the dynamics below a staff. Here I give you the whole reply from the support team:
Technically the dynamic markings do not use velocity in Notion for adjusting volume. We use envelopes within the playback engine for it to be as organic as possible. Of course, it is possible that recorded velocities in your score can cause this issue. Try pressing the "Tab" key on your computer keyboard to view velocities. If you see colored lines, highlight your score and go to Tools>Clear Velocities. That is if you want to clear your velocities. Recorded MIDI velocities can over-write the dynamics.
Another reason for this issue is if you are not putting all of your dynamics below a staff. Once you place a dynamic above a staff, from that point on, you will not be able to put velocities below a staff. They need to all be below or all be above a staff. Sometimes, it is possible to accidentally place a dynamic above a staff that is below the staff that you intend to put it on. If you have any problems, please feel free to send us a score and we would be happy to assist.