Originally Posted by jacksbike

I am assuming that you have a Shimano STX long cage rear derailleur, paired with SunTour friction shifters. When the chain is on the largest rear sprocket and the largest front sprocket, the rear derailleur generally should be at a 45 degree angle. It sounds like you need to put 4 links in your chain. If the chain is too short, the upper pulley wheel, as detailed in the above post, will jam up against the largest rear sprocket and not allow the rear derailleur to drop down . Otherwise, there is also a B adjuster screw on the rear derailleur that moves the derailleur closer or further from the sprocket that it is shifted into. You could also try working with that, moving the derailleur either closer or further from the sprocket. I would try increasing the chain length first.