All the Messiers should be visble in a 12", although a couple (M74 especially) can be tricky. Mone of the northern ones can be hard. In theory M51, for example, should be visible in Vic, but I have yet to nab it here even in the 20". Hopefully being further North you will get a few of those hard to get Messiers!
Alex's suggestion of using brighter objects to check the accuracy of your procedure is a good one.
The important things to think about with manual setting circles on a dob is that the base need to be REALLY level and if using an inclinometer it must be very accurately aligned. A 25mm plossl in your scope wil give (if I remember correctly) about a 0.8deg FOV so if you are 0.5degs awat from your target it will not be visible.
Can I suggest getting a copy of O'Mearas book on the Messiers ( see
http://www.bintel.com.au/Accessories...oductview.aspx) It is an excellent intro to these objects and has hints for observing finding all of them.
Malcolm