Are you using the correct primary index (the second last node of the OID)? This node corresponds to the message memory type. For changeable messages the index should be 3 or 4.
You can retrieve the number of messages for the memory type, (for example, for changeable messages use dmsNumChangeableMsg - 1.3.6.1.4.1.1206.4.2.3.5.2.0) and then the last node of your OID should correspond to the message number in that type of memory bank.
EXAMPLE:
For the first message in changeable memory: 1.3.6.1.4.1.1206.4.2.3.5.8.1.3.3.1
For the second message in volatile memory: 1.3.6.1.4.1.1206.4.2.3.5.8.1.3.4.2