Hair growth is influenced by more than a few factors–some of which you can read about here. But if all else is going well (think: scalp care, strand hydration, etc.), and you can tell your hair is growing from the root but not gaining any actual length, it may be due to unhealthy ends.
Even if your tips aren’t cluttered with splits, dry and brittle ends aren’t going to encourage visible growth. You can certainly use a hair oil to mask those frizzy, dead ends, but once you’re past the point of true damage, there’s no reversing it–best to start over with a fresh trim.