Books online states these restrictions:
Because the DAC exists solely for diagnosing server problems in rare
circumstances, there are some restrictions on the connection:
To guarantee that there are resources available for the connection,
only one DAC is allowed per instance of SQL Server.
If a DAC connection is already active, any new request
to connect through the DAC is denied with error 17810.
To conserve resources, SQL Server 2005 Express Edition does
not listen on the DAC port unless started with a trace flag 7806.
The DAC initially attempts to connect to the default database
associated with the login. After it is successfully connected,
you can connect to the master database. If the default database
is offline or otherwise not available, the connection will return
error 4060. However, it will succeed if you override the default database
to connect to the master database instead using the following
sqlcmd –A –d master
We recommend that you connect to the master database with
the DAC because master is guaranteed to be available if the
instance of the Database Engine is started.
SQL Server prohibits running parallel queries or commands with the DAC.
For example, error 3637 is generated if you execute either of the
following statements with the DAC:
Only limited resources are guaranteed to be available with the DAC.
Do not use the DAC to run resource-intensive queries (for example.
a complex join on large table) or queries that may block.
This helps prevent the DAC from compounding any existing server
problems. To avoid potential blocking scenarios, if you have to run
queries that may block, run the query under snapshot-based isolation
levels if possible; otherwise, set the transaction isolation level to
READ UNCOMMITTED and set the LOCK_TIMEOUT value to a short
value such as 2000 milliseconds, or both. This will prevent the DAC
session from getting blocked. However, depending on the state that
the SQL Server is in, the DAC session might get blocked on a latch.
You might be able to terminate the DAC session using CNTRL-C but
it is not guaranteed. In that case, your only option may be to restart SQL Server.
Do any of these match your case ?