What synth? Assuming you're using a Simple Instrument track, right-click and split it so you can see the MIDI event velocities in the MIDI track, and the audio output of the synth in the synth track. If the event velocities are low, and the MIDI track doesn't have a negative Vel+ value set, then the keyboard is probably sending low velocities.
If the velocities look reasonable, and the audio output level is low, then probably the MIDI track or Keyboard Master Volume control has lowered Master Volume (CC7), or some controller event in a MIDI clip is sending a low value for Expression (CC11) or possibly some other controller that affects output level (e.g. Modulation (CC1) in Aria/Garritan synths).