To note that (big summayr): due to the impact on the cost valuation of a product, the reset to draft button is hidden if the product category is “FIFO or AVCO” and, therefore, there's a difference between the purchase cost and bill price.

Check the user's access rights and make sure they have permissions to edit/delete all relevant bills. The rights may be restricted to only certain bill types or categories.

Hi, I am facing the same problem even though the setting  "Lock Posted Entries with Hash?" is false. Also the hash for the particular invoice is false. What else could cause this behaviour? At about 90% of vendor bills I can find reset to draft but I am finding more and more where I cannot find that option.

Hi, if this problem solved? because we are having the same problem in odoo16, and this doesn't have any problem if the product is set as consumable item.

Check journal setting on vendorbill: if "Lock Posted Entries with Hash?" is set to true, you are not able to set-to-draft.

The reset to Draft only shown for user has group (Billing, Bookkeeper, Accountant) also if the bill status is (posted or cancelled) and the "Lock Posted Entries with Hash?" is set to False in the Journal Advance Settings (Vendor Bills Journal). 

Are you have multi journals with purchase type?

Also if you are in automatic inventory valuation and there is a stock valuation layer to correct the prices.