This might be horribly wrong, but when I've hit a bulky seam that won't open properly (usually because I haven't trimmed it well and there's more fabric than it's supposed to have) I put a little bit of spray starch into a shot glass and use a paintbrush to wet the seam from the wrong side. Then I drop the iron onto it and press (usual story, DON'T wriggle it around, just press down). The moisture relaxes the seam and lets it open, then the starch sets it so it can't shift back up when it dries.