If you are facing issues checking out multiple products in one order, it is likely due to the following reasons:
If the shipping options for the products are set differently, you would not be able to check out all the products under the same order / transaction.
Each Shopee Integrated Shipping channel has its own weight and/or dimension limits per order/shipment. Therefore, if you are trying to check out multiple bulky items under one transaction, you might not be able to proceed with the order, unless the seller also offers non-integrated shipping channels for the products.
You can only check out a maximum of 40 different products per order. Sellers can also limit the number of products each buyer can check out per order. Make sure that you have not exceeded these limits.