Why is Professional Development so hard to do successfully 1

We all have had to sit through some expert talk about a topic that your school or principal determined was important for the success of students. Usually this is because the school looked at the data and saw that the students struggled in one particular area (usually writing, math, or reading....it's funny how it is never science or social studies, or physical education because half the students and 75% of the staff is overweight!) Enough about that rant, the fact is that professional development is broken in most schools.  I would argue all schools, but I do not know how all schools function and if they found the magic spell to make professional development work.  One of the buzz words around the educational arena is "differentiated instruction."  You know delivering the curriculum in a way that all kids will understand (how's that working for you?).  The idea is great, but would really only work for very small group learning or ideally 1 to 1 learning.  So how does this relate to professional development?  PD has to be individualized to meet the needs of each teacher, otherwise it can be irrelevant.  And if it is irrelevant, then as a school you are wasting the time of your teachers (or some of them).  This is where online PD can be useful...to be continued