If it happens after the second factory, maybe you don't have enough industry and it's not getting delivered because there's nowhere for it to go? I usually force industry to stay at medium density. 1 Omega factory and 1 Robotics factory fits exactly for 7 medium density, medium tech industrial buildings. Pretty important to get your tech up to medium (use community colleges) and make sure you have zero unfilled freight orders.
So if you draw a straight line with a dirt road that costs 1440, then draw another one that is 440 away from the first one, you can fit them. So you have a rectangle that is 1440 x 440 - 7 MD industrial buildings will fit on one side, and 2 Omega factories will fit back to back with them on the other side. You'll even have room between the factories to plop a ground scrubber if you have unlocked them. Make one of the factories a full robotics line and the other a full omega line.
Having all medium density buildings makes more money from Omega than high density.
I'll try to remember to grab a screenshot of one of mine to explain what I mean