Sandpaper is more work than repeated stripper applications...plus, with sanding, you risk sanding into good wood around the stuck-on paint.
If there are large areas of resistant paint, you can use an orbital wallpaper scorer ($10-$15) to help the stripper sink in...or just take a razorblade and lightly cut into it..."scoring" it.
Finally, I hope you're only doing the exteriors. On the interiors, just sand and paint over the old paint.
No one will ever notice
