I really am not sure why teachers should have trouble knowing how to add, subtract, multiply, divide and maybe some work with fractions.  This is what students mostly learn from grades 1-6, and which they're apparently getting worse at doing.  Since these are absolutely basic tools before you can do anything else in math, that's pretty bad.  But a teacher shouldn't require special math skills to teach arithmetic at this level. 

What I HAVE heard is that teachers don't understand how to teach the discovery math stuff, since it's not straightforward.  Rather than re-educating the teachers on that, I'd chuck the curriculum and focus on those basics. 

Later on, algebra, trigonometry and calculus require better honed skills obviously, but this is not the issue with little kids.