IDE not checking out

We have not been able to figure out the cause of this yet, but was
hoping someone else has had this problem.  We have a developer working
offsite using Vault.  He has VS.Net setup to automatically checkout a
file on edit.  However sometimes, the file does not get checked out, but
is allowed to be edited.  Has anybody else seen this before.  I really
don't know which end of the spectrum is causing this.  The on-site
developers have never had this problem before, so it's a little strange.
We keep on losing changes because of this, which is annoying.

Lance Johnson
Software Developer
DOCS, Inc.
(800)455-7627 x401