I don't think there is anything you can do to increase the gravity of a beer that is already brewed, and I'm not going to pretend that I know what effect it will have on the beer that you have brewed as I have no experience of Bocks, and I don't like Dunkels.
However, there are a number of things you can do to determine the cause, and prevent it happening in the future.
What you need to do to prevent this is to take gravity readings during and after the sparge, and then to adjust the gravity to the required value with malt extract during the boil.
Assuming that you are batch sparging, you should take a gravity reading for the runnings after each addition of sparge water. The gravity of the runnings after each addition should be less than the previous batch, and the gravity of the final runnings should be between 1.010 and 1.020. If you are much above 1.020, then you have sparging problems that are negatively affecting your efficiency.
If the final runnings are not significantly above 1.020, then any efficiency problems are not sparge related. Look to mash temperature, grain crush, and water/grain ratio for solutions.
After the sparge is completed, you should stir your wort really well and then take the gravity of your wort. The pre-boil gravity multiplied by the pre-boil volume divided by the post boil volume will give you a pretty good approximation of the OG.
The main problem with gravity readings is that they are very temperature sensitive. I use a refractometer to take the readings. As the sample size for a refractometer is only one drop, temperature is no longer an issue. You can use http://byo.com/feature/1132.html
to give you information on how to do this manually, or get brewing software such as Promash to make the conversions automatically.
I hope this makes sense, but I've spent a lot of time this evening comparing two high gravity brews. I really must learn not to swallow.