Of VSTs and loudness

Hello everyone,

I've been making a few tracks using mostly virtual instruments (vst), samples and my own voice.

I didn't have any problem until I decided to master my tracks. The VSTs are incredibly quiet compare to the samples. I didn't noticed it when I did the first mix. The volume in the VST is at the maximum, and boosting the decibels just fuck up the sound with clipping.

Thus result in my mix not being loud enough to send it to mastering. It works but it's not just loud enough.

TLDR: I don't know if I'm clear enough but VSTs volume are way too low campare to any other media imported. Does any of you had this issue ? How can I fix it?

I work on Cubase Elements and the VSTs are HALION and a whole bunch of .sfz

Thanks a lot !