How long has it been producing CO2?
What are you using as a diffuser?
How did you make your system, what did you use for parts?
If the diffuser isn't providing much of a restriction, there won't be much pressure. This isn't necessarily a bad thing; there is less risk of a rupture in the system. If you are still getting a sufficient amount of CO2 into the tank with small bubbles, that is the only thing that really matters.
There could also be a leak somewhere in the system. Most likely where the airline goes into the fermentation container.
More yeast will also ferment faster requiring more frequent changes. Some people find this to be a pain.
Cooler temperatures will cause a slower fermentation.
The types of sugars and available nutrients will effect the fermentation rate.
If the system is new and has never fermented well through multiple batches, I would check the setup for leaks.