Help with Work & Insurance Claim - Keeps getting denied!

Like some others out there, my husband is having a hard time at work.  His ADD combined with anxiety & depression have left him off work at the moment.  He has lost a job in the past due to all of this as well.  He just cannot seem to cope.  He is about to lose his current job as well.  He has been denied disability through his insurance company.  We are so frustrated & at a loss.  Has anyone else experienced this?  The insurance company keeps saying to send in more documentation.  How much more do they want?!  Unfortunately Dr.'s are not always the bast at writing letters for this either.  Any advice?  Thanks.