If you're describing what I think, then I also had that problem with a Scarlett 2i2. The input level was very low when recording with a dynamic microphone, even with the input level on the Scarlett at max. (However, there wasn't any problem when using a condenser mic with the phantom power switch engaged.) I was told on various forums to buy a Cloudlifter to boost the signal from the dynamic mic. In the end, I figured it was still recording at a sufficient level when the input gain was maxed, so I didn't get the Cloudlifter. Monitoring with headphones wasn't a problem because I could amplify the output signal through the instrument track in my DAW.