Books online states these restrictions:

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

command:

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:

RESTORE

BACKUP

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 ?