i was programming movement through the event sheet (not using behaviors) so the solid behavior didn't work: things went right through. perhaps the dev team will fix that next release.
This is by design! Only behaviors react to Solids automatically. Each behavior reacts differently to solids. If you move objects by events, Construct 2 doesn't know what kind of movement it is, so it doesn't know how to respond to hitting the solid. If you move objects by events, you also have to handle collisions with solids by events.