Significant dropoff from initiating check out to completed. Thoughts on why?

Hi - My business partner and I run a small apparel company. We're seeing good "add to cart" and "initiate checkout" rates from our ads and email campaigns.

However, we're seeing a 75% drop from that point to actual sales. Example from a recent email campaign:
* 150 adds to cart

* 105 began checkout

* Only 27 converted to sale

We've done troubleshooting on the site (no issues) and our pixeling appears correct. Anyone else experiencing a drop like this and if so, any suggestions on what might be going on that might be related to Shopify?

Thanks for any help you can offer!