I think there is some confusion here concerning the fertility of hybrids. Hybrids are not sterile. If hybrids were sterile, then there would be no breeding past primary hybrids. Hybrids are bred with hybrids all the time with great success, and at an amazing rate. Just check out the latest Sander's List of all the Phal hybrids registered in the last couple of months.
Sterility becomes an issue with aneuploid plants, those with an uneven number of sets of genetic material. All naturally ocuring Phal species (except one) have two sets of genes - one from each parent (2n or diploid). But many modern lines have double the chromosomes (4n or tetraploid). If a diploid is crossed with a tetraploid, the resulting plant would be triploid and have sterile pollen. Triploids can (unfortunately) be used as the seed parent, making matters worse. Although triploids are out there in droves, odds are that any particular plant is either 2n or 4n, and therefore perfectly fertile.
Long story short: just because two plants can breed, doesn't mean they should. If you're getting into breeding, learn the basics of ploidy to avoid creating more breeding dead-ends.
All that being said, some species and their offspring can be reluctant breeders requiring perfect conditions to commit to growing a capsule. Some won't breed until they're all grown up. Some will breed like crazy. If a cross doesn't take it may just not have been ready to put energy into making seed. Just my two cents.
Jeffery 
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