Try different ad units to start, because FB serves diff creative types to different pockets of the audience. If you have the budget, you can try doing other objectives, but we've seen that the reason why we don't get much reach on the retargeting on conversions is that FB knows that most of the ppl in that audience aren't ready to convert. So it's very likely if you run other objectives, you won't get any profits at least short term. Gotta be ready for that