What could be causing low volume?

For some reason I am unable to raise the volume levels of my VST instruments in my song beyond about -25db (digital scale). Main stereo output is at 0db. No sub groups. Nothing on main bus. Please give me some things to check that could be causing this ceiling. Thanks

Maybe you accidentally added volume automation, or something in the Controller Lane of the key editor that kicks it down at the beginning of the timeline.

Where are you trying to adjust the level, on the Cubase Channel or a Volume Control internal to the VSTi.

Also there is a Test Tone Generator plug-in you might find useful

Channel Pre-Gain?

What instruments would those be? If you load a Groove Agent SE instrument, does it still do it? Some instruments expect CC1 (or other CCs) for dynamics. If the modwheel is completely down, the sound would be weak.