How to increase the gain on a preamp

The question cannot be answered without knowing what the preamp model and cartridge are.

Is it possible that you are running a low output MC into a MM or low gain phono stage? If so, you may need to use a Step Up Transformer (SUT) for your phono.

Use MM cartridge instead. It's way cheaper than SUT.

Agree with Onhwy61 and Jmcgrogan2. It sounds like you may not have enough gain in your MC stage to properly amplify the signal from your cartridge, but we'd need to know how much gain you have in the phono stage and the output of the cartridge. An SUT (which I have used in the past and found very effective, but a bit pricey) or a higher output cartridge may be in order.

What Onhwy61 said.