Amazon will not allow products to be shipped to their warehouses that they consider being meltable between April and October months.
More information on this topic and a list of meltable ASINs can be found on Amazon's help page:
Meltable FBA inventory
Note: You will not see this type of restriction immediately when searching for products on Seller Central's "Add a Product" page because you still have the option to sell them as Merchant Fulfilled. In order to check in Seller Central, you would need to attempt to send the product as FBA by adding it to a shipping plan.