Why do some individuals who are productive PhD students at leading Western institutions, then publish infrequently as faculty in their home country?
The problem that faces faculty at second- (and lower-) tier institutions including those in foreign countries is that research is often not valued or not given the resources necessary to flourish.

For example, at many national universities  faculty have considerable responsibilities other than research, including teaching and national and local service. They may not have enough (or any) quality graduate students to help with teaching,  to inspire them to explore new research areas, or to conduct research together.  They may also have much more administrative duties with much less administrative support. 

Finally, there may be little incentive for them to do or publish research. They may have civil service salaries that are not adjusted for research output. External consulting or tutoring may be more lucrative than doing work within the university.