Find the right adapter for 4 D batteries
There are 3 big reasons why a device would use D cells:


Weight/mass (no, really)
Capacity (alkaline D cells start at about 10Ah)
Current draw (alkaline D cells can deliver an ampere easily, nicad several amperes, although Li-ion is starting to take over)


Given that your device has a motor, its reason likely falls somewhere along the last two. As such, your adapter is severely underpowered. I would recommend that you find one capable of delivering at least 2A, with 3.5A preferable to be safe.