Is the extractor completely gone, or broken off? If it's still partially there then it needs to come out before the block will drop. If not, then often times you need to give a little push back on the block as you pull down on the lever. As John mentioned they often hang up from years of dirt accumulation, so it may take some effort and lube to get it to move.
Marlin lever actions 1870's-WWI, Ballards, and single shot rifles!