Well here are just a few points that come to my head on the issue of declining social mobility:
1) Loss of low end labouring jobs - decreases opportunities for those who are less academically gifted.
2) War on drugs - an invention of the 20th century that has provided generations of people with a criminal record restricting their employment opportunities.
3) Ethnic Immigration - Mostly positive affect. However their are those ethnic communities that tend to preference family over community. They expand their business by hiring only people within they're family rather than hiring from the broader community.
4) Casualisation of the workforce - More people are being put into casual jobs rather than full time. This diminishes the benefits they receive and they're ability to apply for any kind of bank loan.
5) Greater education requirements - More and more jobs require post high school qualifications, further squeezing out people already affected by 1).
I know there are other factors but that's all I can think about now.