When you sync with Amazon you can only sync with your own Seller Central Account (check and you should see this is definitely in sync after allowing time for Amazon to process).
Because multiple sellers are sending multiple different images, titles, descriptions etc. for the same product it is at Amazon's discretion which description they choose to use. Even if you are selling a product unique to you, Amazon may not decide to update the front-end listing. This is rare and usually they will after a day or so but not always. You can try contacting Amazon support in these cases.
Marketplace Connect syncing with Amazon Seller Central can be affected by the configuration settings you choose, for example if you have configured a product to have a different price on Amazon to that in Shopify (either using a fixed price, a pricing operator or an attribute mapped custom Amazon field price) this could be the reason why your Shopify product doesn't reflect what's in Seller Central.
If you believe there is a sync problem, please reach out to email@example.com