It could be a few things. First check that the diesel return flow pipe coming off the injectors isn't blocked. To do this, with the engine running pull off fuel pipe where it goes back to the pump there should be diesel coming from there. If not then the leakback pipe nuts have been overtightened.
Second it could be the o rings on the fuel pump are dead (are you using bio-diesel?)
If not the first then most likely the second solution.
If not these them maybe injectors blocked but unlikely because it is running.
Hope this helps.