I ran across an interesting issue in the Autodesk Discussion Forums, and thought I’d re-post it here in case someone else had the same trouble.
The poster was confused as to why blocks inserted from a tool palette didn’t seem to obey object snaps during placement, despite the fact that they were active.
A little detective work uncovered that the methodology was to blame—if you click-and-drag a blog from a palette into a drawing, object snaps get a little twitchy. (Is that enough of a technical term?)
However, if you click-and-release on a block, then you can place it with object snaps just like in any other insert or modification command.
The poster complained of this issue in 2011 and 2012, but it seems to be better in 2013. All the same, I recommend single-clicking over click-and-drag from tool palettes.