Sometimes if you do get a proper charge off with all your units lines up and in formation, they still won't lower their spears. The most reliable way I've seen to get a charge is to run them in, and then hit 'r' when they are in charging distance. This puts them out of run, into walk, and the combat AI will make them lower their lance. This is because running doesn't seem to reliably trip the lower lance AI routine while walking does.