Lego have issues a tecnicle builitin for the 2008 Radio Controled Bulldozer
It would appear that one of the axles has a tendancy to be left sticking out of where it should be - this is probably due to a the axle moving by half a "brick" in one of the instruction steps.
This document from the Lego site points out where the axle has been found to be sticking out. This causes the front blade of the Bulldozer and the Rear Ripper not to raise and lower if the problem isn't fixed. Unfortunatly you'll have to part dismantle your model to fix it.