There is another thread about the spears being less than effective against ... anything.

I've found that 4 ranks deep don't cut it like they used to in MTW/RTW and now set them up with 6 ranks deep at least.

Sometimes the units front row in a charge will run and leave the rest of the unit behind in a sort of defensive fighting stance, this is not specifically a spear unit issue.

Try using the schiltom (sp?) formation, I've ripped apart king's generals bodyguards with 2 spear militia in front of a gate before.

I'd also suggest that the first instant you can get any better troops you leave the spearmen on the flanks, better than cavalry plugging the gaps. I tend to have dismounted knights as my center and a levy spear at either end with billmen or dismounted english knights slighty behind ready to manouver to their flanks. I'm in an English campaign (obviously from the units I'm describing) and there are few decent spear units in the arsenal, for Italian factions/HRE etc it may be different.