You're running into compression and EQ problems. The digital domain is very unforgiving when it comes to clipping. You'll need some master compression for starters. This will tame down the peaks so you can turn the overall volume up further. The more compression, The louder the cd, but also the less dynamic range. You will have to decide how much dynamic range you will give up for a louder CD.
EQ is also a factor. Listen to commercial recording and compare them to yours. Do yours seem to have a lot more bass, or treble, or mids? If so, you have limited your perceived overall volume again because the overabundant frequency is putting your recording at top levels. If this frequency/frequencies were brought back some, you could bring the overall volume up.

Or maybe you have the opposite problem; I.E. most of the frequencies are correct, but a few are too low. Therefore, the sound is thinner and perceivably quieter because some of the spectrum is missing.

Hope that makes some sense to you.