Question about cycling with LR

[Llamarama]

Hiya!~
This might be a bit of a noob question but having dealt with only FW i'm not sure what the difference is.
We put together a 25Gal tank filled it with sand and SW and added about 20 LBS of cured live rock to start with and get the cycle going....
It's been 3 weeks and ammonia is reading 0 but nitrite is 5ppm + and nitrate is 0
Could there be some reason the bacteria turning nitrite to nitrate is not growing? is there a way to speed it up?