We are having this problem roaming around our office. some files it seams like a feature and others files come and go. multiple machines, multiple users.
if you sketch in any entity and insure it has no constraints you are unable to drag it on your sketch plane. you can constrain it or dimension it but to simple drag it it does nothing. we have tried most things that people have suggested and to no avail.
I will attach the file to see if it does it for you all.
SW 2011 SP5