Theoretically with 12 skills , lets say on average 10 skills per training type can be effected by cross training , and at 10 percent training going to cross training. over 14 weeks its highly likely that you can be unlucky because of low sub levels throughout all skills , that nothing can pop in a season. The same goes to for 2 seasons but any longer than 3 seasons and a pop must be due. Perhaps all of your skills are at very high sublevels and you will get a fair few amounts of random cross training pops in a row.