The wiki has this to say about CMC generation:
Civilian Mining Colony Generation
A Civilian Mining Colony will only generate after a number of conditions are met:[1]
1 First, a random number is generated between 1 and 1,000,000. That random number is compared to annual empire wealth generation. If the random number is smaller than the annual empire wealth, the next generation step occurs. Otherwise, no new CMC will be generated that production cycle.
2 Next step is to determine which system a body will be generated in. Each System with more than 10m population is ranked from most population to least. The most populous system has a 50% chance to go on to the next step. If that fails, the next populous system is given a 50% chance, down the line. If all systems fail the chance, the CMC will be generated in the largest population system.
3 Once a system has been chosen, a body must be chosen. First, the body must be surveyed and have no existing colony on it. It needs to orbiting the same star as the selected population, and be no more than 80AU away.
4 On the first run through those bodies that qualify, Aurora checks for deposits of either Sorium or Duranium in excess of 25,000 tons with 0.8 accessibility or greater.
5 If no bodies qualify, Aurora then checks for deposits of either Sorium or Duranium in excess of 15,000 tons with 0.7 accessibility. A tonnage check similar to 4B is performed to evaluate which body gets the CMC.
6 Once either step 4 or 5 find at least one candidate, Aurora finds the total mineral deposits for each body. The deposit size is multiplied by accessibility, ignoring all deposits of 0.4 or less.
7 The world with the greatest total deposit is selected and a CMC of size 1-3 is created on that body
So it looks to me that the limiting factors for CMC generation are :
1) empire wealth must be below 1,000,000
2) Bodies in the selected system need to have either sorium or duranium with above .7 accessibility
3) Those bodies must be surveyed and have no colony on them
If your planets don't quife qualify you might try using spacemadter to fudge the numbers a bit, maybe cram some duranium on them, you could make up for it by taking 15,000 duranium from another planet you were intending on mining if you want, or even take it from your stockpile.
