The alien ship appears to be stranded and inoperable for the 20 years it hangs over Johannesburg. Early on we are shown that a small vessel had detached from the ship and disappeared.
Approaching the movie's climax we learn:
1. The small vessel was hidden under a shack in District 9 the entire time.
2. The vessel is a command module that can activate the alien technology and the stranded ship itself.
3. Alien Christopher Johnson (and cohorts) spent 20 years collecting enough "fluid" from alien scrap metal to power up the command module ... and (he suggests) to subsequently get the entire ship very quickly to retrieve alien help.
All of the "fluid" needed to power the ship to leave Earth came with the ship itself. In fact, enough could be extracted just from the small pieces of debris that fell off of the ship onto the ground while it was stranded.
Even if we accept that the ship lost or ran out of the distilled fluid fuel, is there a good explanation for why Christopher Johnson didn't stay on the ship and extract it there directly, rather than going to ground to scavenge it from scraps and distill it in a shack over the course of 20 years?