CentPrime review for Trendsi: Fashion Dropshipping

I have a Trendsi account and have installed the Trendsi app on my Shopify store. A customer recently ordered a pair of pants in size Medium from my store, but when I checked the product on Trendsi, there was no Medium stock available and only 2 units left in size Small. On my Shopify site, however, the same pants still show more than 50 units available across sizes S, M, and L. Because of this discrepancy, I now need to refund the customer and explain that the item is actually out of stock. After further checking, I discovered that for more than half of the products imported from Trendsi, the available variations, colors, and sizes in Shopify do not match what is shown for the same items on Trendsi. This indicates that the Trendsi app is not syncing inventory and variants with my Shopify store in real time. Please explain why this syncing issue is occurring and provide a solution so that my Shopify inventory always matches Trendsi and this situation does not happen again.