This could be due to a few reasons:
When you install the app, make sure that "Shopify tracks this product’s inventory" is selected under Inventory Policy in order for the sync to work
You must also be opt-ed in for Dropshipping. You can find this input under Edit Profile in the Dropshipping tab
The inventory changes are only reflected after a product has been paid for by the Retailer
If all of the above doesn't apply to you, please e-mail support@modalyst.co and we will help you out right away.