Or, to put it more charitably, medicine and psychology are far describing far more complex phenomenon than we like to admit.
For example, in psychiatric genetics, there are dozens of articles every year that find a new gene associated with a common and important condition (e.g. autism, schizophrenia, depression). After each new finding comes out, there are dozens of labs that try to replicate that finding, usually one or two replicate (or partially replicate) the finding, and five or six don't replicate it. Why is it so hard to replicate these findings? Probably because there are really dozens of independent genes that contribute to these complex disorders (probably in combination with each other), and some populations tend to have mutations in one set, while other populations tend to have mutations in another set.
We're moving towards understanding, but the disorders are far more complex than the assumption that there will be a single cause.
Also, there's not that much literature on RGS14 at this point (it doesn't seem to have come up in any of the GWAS -- wide scale genome association studies) for psychiatric disorders, but it has been identified in molecular studies as a target of P53 (a central cancer regulatory mechanism). It would not be out of the question for this knockout to have a significant increase in cancer risk (brain or elsewhere), but not have this detected in a small-scale study.
We cannot command nature except by obeying her. -- Sir Francis Bacon