Well...if there is maybe a part missing and it still does not work I would suggest taking it apart, checking if you have all the pieces and going from there. Much simpler that way than guessing I have learned. Make sure all caps and pieces are tight, oftentimes my xp3 has small issues after taking it apart and everytime it is because a piece of the intake is loose, or the prime cap is not sealed on tight enough, or there is air in the canister itself.
Does it work at all? Like said above real words and pics or better details would help you out.